Faculty of Medicine Introduction to Community Medicine Course
(31505201)
Unit 4 Epidemiology
Introduction to Epidemiology Definition History of Epidemiology PurposeUse of Epidemiology
Concepts in the infectious diseases
By
Hatim Jaber MD MPH JBCM PhD
20+23 -10-2016
Presentation outline
Time
Introduction to unit 4 Epidemiology 1200 to 1210
Public Health amp Epidemiology 1210 to 1220
Key Words 1220 to 1230
Main components 1230 to 1240
Concepts in the infectious diseases
Introduction to unit 4 Epidemiology bull Definition History of Epidemiology
PurposeUse of Epidemiology
bull Disease Causation
bull Levels of prevention
bull Concepts in the infectious diseases
bull Measurements of Morbidity and Mortality
bull Sources of Data and methods of data collection
bull Epidemic Investigation and Management
bull Epidemiological Surveillance
Epidemiology
bull Epi = upon
bull Demos = population
bull Logos = study of
Definition
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Last 1988
Epidemiological Principles
bull bullDiseases (or other health events) donrsquot occur at random
bull bullDiseases (or other health events) have causal and preventive factors which can be identified
bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than
individual persons tissues or organs
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Presentation outline
Time
Introduction to unit 4 Epidemiology 1200 to 1210
Public Health amp Epidemiology 1210 to 1220
Key Words 1220 to 1230
Main components 1230 to 1240
Concepts in the infectious diseases
Introduction to unit 4 Epidemiology bull Definition History of Epidemiology
PurposeUse of Epidemiology
bull Disease Causation
bull Levels of prevention
bull Concepts in the infectious diseases
bull Measurements of Morbidity and Mortality
bull Sources of Data and methods of data collection
bull Epidemic Investigation and Management
bull Epidemiological Surveillance
Epidemiology
bull Epi = upon
bull Demos = population
bull Logos = study of
Definition
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Last 1988
Epidemiological Principles
bull bullDiseases (or other health events) donrsquot occur at random
bull bullDiseases (or other health events) have causal and preventive factors which can be identified
bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than
individual persons tissues or organs
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Introduction to unit 4 Epidemiology bull Definition History of Epidemiology
PurposeUse of Epidemiology
bull Disease Causation
bull Levels of prevention
bull Concepts in the infectious diseases
bull Measurements of Morbidity and Mortality
bull Sources of Data and methods of data collection
bull Epidemic Investigation and Management
bull Epidemiological Surveillance
Epidemiology
bull Epi = upon
bull Demos = population
bull Logos = study of
Definition
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Last 1988
Epidemiological Principles
bull bullDiseases (or other health events) donrsquot occur at random
bull bullDiseases (or other health events) have causal and preventive factors which can be identified
bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than
individual persons tissues or organs
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemiology
bull Epi = upon
bull Demos = population
bull Logos = study of
Definition
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Last 1988
Epidemiological Principles
bull bullDiseases (or other health events) donrsquot occur at random
bull bullDiseases (or other health events) have causal and preventive factors which can be identified
bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than
individual persons tissues or organs
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemiological Principles
bull bullDiseases (or other health events) donrsquot occur at random
bull bullDiseases (or other health events) have causal and preventive factors which can be identified
bull Diseases and health have a distribution bull Epidemiology focuses on populations rather than
individual persons tissues or organs
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull laquo The art of epidemiological thinking is to
draw conclusions from imperfect data raquo
George W Comstock
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemiology and Preventive Medicine
bull Epidemiology is the central science of public health
and preventive medicine is a clinical approach to public health practice Epidemiology can provide the preventive medicine practitioner with the following
bull - Information on the state of the health of the population
bull - Methods for identifying possible determinants of health and disease within individuals
bull - Appropriate population groups for interventions
bull - Understanding the origins of public health recommendations
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Why have chronic diseases increased in importance
bull Epidemiologists investigate both infectious and chronic (non-communicable) diseases
bull Favorable demographic changes and public health successes during the 1900rsquos (quality and availability of food water housing sanitation communicable disease control)
bull bullldquoEpidemic transitionrdquo (ie change) bull ndashLower overall death rate bull ndashGreater life expectancy bull ndashShift in major causes of death (from infectious to non-
communicable) bull ndashChange in lifestyles (access amp economy)
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Defining characteristics of chronic diseases
bull bullProlonged course of illness
bull bullMultiple risk factors
bull bullLong latency period
bull bullNon-contagious origin (non-communicable)
bull bullUncertain etiology (causation)
bull bullFunctional impairment or disability
bull bullIncurability
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Distribution Time place person
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Key Words
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Determinants Cause risk factors
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Population = Public health
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems
bull Application = Information for action
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Major components of the definition
bull 1 Population The main focus of epidemiology is on the effect of disease on the population rather than individuals For example malaria affects many people in Ethiopia but lung cancer is rare If an individual develops lung cancer it is more likely that heshe will die
bull Even though lung cancer is more killer epidemiology gives more emphasis to malaria since it affects many people
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Major components of the definition bull 2 Frequency This shows that epidemiology is mainly a
quantitative science Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions Frequency of diseases is measured by morbidity and mortality rates
bull 3 Health related conditions Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health Health related conditions are conditions which directly or indirectly affect or influence health These may be injuries births health related behaviors like smoking unemployment poverty etc
bull 4 Distribution Distribution refers to the geographical distribution of diseases the distribution in time and distribution by type of persons affected (where When and who)
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Major components of the definition
bull 5 Determinants Determinants are factors which determine whether or not a person will get a disease
bull 6 Application of the studies to the promotion of health and to the prevention and control of health problems This means the whole aim in studying the frequency distribution and determinants of disease is to identify effective disease prevention and control strategies
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Natural history of disease
bull The ldquonatural history of diseaserdquo refers to the progression of disease process in an individual over time in the absence of intervention
bull There are four stages in the natural history of a disease These are
bull 1 Stage of susceptibility
bull 2 Stage of pre-symptomatic (sub-clinical) disease
bull 3 Stage of clinical disease
bull 4 Stage of recovery disability or death
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull 1 Stage of susceptibility bull In this stage disease has not yet developed but the groundwork has been
laid by the presence of factors that favor its occurrence bull Example unvaccinated child is susceptible to measles
bull 2 Stage of Pre-symptomatic (sub-clinical) disease bull In this stage there are no manifestations of the disease but pathologic changes
(damages) have started to occur in the body The disease bull can only be detected through special tests since the signs and symptoms of the
disease are not present bull Examples bull bull Detection of antibodies against HIV in an apparently healthy person bull bull Ova of intestinal parasite in the stool of apparently healthy children bull The pre-symptomatic (sub-clinical) stage may lead to the clinical stage or may
sometimes end in recovery without development of any signs or symptoms
Four stages in the natural history
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
3 The Clinical stage
bull At this stage the person has developed signs and symptoms of the disease The clinical stage of different diseases differs in duration severity and outcome The outcomes of this stage may be recovery disability or death
bull Examples bull 1048707 Common cold has a short and mild clinical stage and almost
everyone recovers quickly bull 1048707 Polio has a severe clinical stage and many patients develop
paralysis becoming disabled for the rest of their lives bull 1048707 Rabies has a relatively short but severe clinical stage and almost
always results in death bull 1048707 Diabetes Mellitus has a relatively longer clinical stage and
eventually results in death if the patient is not properly treated
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
4 Stage of recovery disability or death
bull Some diseases run their course and then resolve completely either spontaneously or by treatment In others the disease may result in a residual defect leaving the person disabled for a short or longer duration Still other diseases will end in death
bull Disability is limitation of a persons activities including his role as a parent wage earner etc
bull Examples
bull bull Trachoma may cause blindness
bull bull Meningitis may result in blindness or deafness Meningitis may also result in death
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
1 Describe the health status of population
2 Explain the etiology of disease
3 Predict the frequency and distribution of disease
4 Control diseases in populations
Aims of Epidemiology
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Uses of Epidemiology
bull Determine the magnitude and trends
bull bullIdentify the etiology or cause of disease
bull bullDetermine the mode of transmission
bull bullIdentify risk factors or susceptibility
bull bullDetermine the role of the environment
bull bullEvaluate the impact of the control measures
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Uses of Epidemiology bull To make a community diagnosis Epidemiology helps to identify and describe
health problems in a community (for example the prevalence of anemia or the nutrition status of children)
bull To monitor continuously over a period of time the change of health in a community (for example the effect of a vaccination programme health education nutritional supplementation)
bull To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera)
bull To investigate an outbreak of a communicable disease analyze the reasons for it plan a feasible remedy and carry it out and monitor the effects of the remedy on the outbreak
bull To plan effective health services Effective services interventions and remedies all depend on accurate community data
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemiologist Core Functions
bull Public health surveillance
bull bullOutbreak investigations
bull bullData analysis
bull bullEvaluation of disease control programs
bull bullCommunication
bull bullManagement and teamwork
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemiologic triad
Host
Agent Environment bullBiological agents bullPhysical agents bullChemical agents bullNutrient agents bullMechanical agents bullSocial agents
bullPhysical environment bullBiological environment bullSocial environment
bullDemographic characteristics bullBiological characteristics bullSocioeconomic characteristics
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Host Agent Environment
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Terminology and Definitions
bull Infection bull Contamination bull Infestation bull Contagious disease bull Incidence and prevalence of
infectious diseases bull Epidemic bull Endemic bull Hyperendemic bull holoendemic bull Pandemic
bull Exotic bull Sporadic bull Attack rate bull Primarysecondary cases bull Zoonosis epizootic and
enzootic bull Nosocomial infection bull Opportunistic infection bull Eradication bull Elimination
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Terminology and Definitions (cont)
bull Virulence
bull Reproductive rate of infection
bull Host
bull Vector (source)
bull Reservoir
bull Incubation period
bull Infectivity period
bull Serial interval
bull Latent period
bull Transmission Probability ratio
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Infectious Diseases Epidemiology
Definition
bull Communicable disease (infectious disease) ndash is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person animal or reservoir to a susceptible host either directly or indirectly through an intermediate plant or animal host vector or the inanimate environment
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Components of the infectious process
bull The infectious process of a specific disease can be described by the following components which constitute of the chain of disease transmission
bull 1 The Agent bull 2 Its reservoirs bull 3 Its portal of exits bull 4 Its mode of transmission bull 5 Its portals of entry bull 6 The human host
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Components of the infectious process
bull I The Agents bull The agents in the infectious process range from
viral particles to complex multi-cellular organisms
bull II Reservoirs bull A reservoir is an organism or habitat in which an
infectious agent normally lives transforms develops andor multiplies Reservoirs for infectious agents may be humans animals plants or other inanimate objects
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Types of reservoirs
Reservoir
Human reservoir
Animal reservoir
Non-living reservoir
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Human reservoir
Human reservoir
cases carriers
According to spectrum of disease bullClinical cases (mildsevere-typicalatypical) bullSub-clinical cases bullLatent infection cases
bullPrimary case bullIndex case bullSecondary cases
Type bullIncubatory bullConvalescent bullhealthy
Duration bullTemporary bullChronic
Portal of exit bullUrinary bullIntestinal bullRespiratory bullothers
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Reservoir
bull Habitat in which the disease normally lives and multiplies
bull bullPeople -Symptomatic - Smallpox -Asymptomatic - HIV bull bullAnimals (Zoonoses) -Brucellosis -Plague bull bullEnvironmental -Histoplasmosis -Legionnairesrsquo bacillus
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Components of the infectious process bull Some diseases with human reservoirs are Most bacterial and viral
respiratory diseases HIVAIDSSexually Transmitted Infections (STIs) measles typhoid etc
bull All infected humans whether showing signs and symptoms of the disease or not are potential sources of infection to others
bull A person who does not have apparent clinical disease but is a potential source of infection to other people is called a Carrier An example of carrier is a person infected with HIV A person infected with HIV might not have the signs and symptoms but heshe is capable of transmitting the infection to others
bull Some diseases are transmitted to human beings from animals These diseases are called zoonoses Examples Rabies anthrax etc
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Components of the infectious process
bull III Portal of Exit
bull Portal of exit is the way the infectious agent leaves the reservoir
bull Possible portals of exit include all body secretions and discharges Mucus saliva tears breast milk vaginal and cervical discharges excretions (feces and urine) blood and tissues
bull For example feces is the portal of exit for the eggs of hook worm
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
IV Mode of Transmission
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
IV Mode of Transmission
Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts Transmission may be direct or indirect
bull 1 Direct Transmission bull 11 Direct contact Occurs when there is contact of skin
mucosa or conjunctiva with infectious agents directly from person or vertebrate animal via touching kissing biting passage through the birth canal or during sexual intercourse Example HIVAIDSSTIs rabies
bull 12 Direct Projection is transmission by projection of saliva droplets during coughing sneezing singing spitting or talking Example common cold
bull 13 Transplacental is transmission from mother to fetus through the placenta Example syphilis HIVAIDS
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Indirect transmission bull 2 Indirect transmission The following are the different types of indirect
transmission
bull 21 Vehicle-borne Transmission occurs through indirect contact with inanimate objects fomites bed sheets towels toys or surgical instruments as well as through contaminated food water IV fluids etc
bull 22 Vector-borne The infectious agent is conveyed by an arthropod to a host Vectors may be biological or mechanical
-Biological vector A vector is called biological vector if the agent multiplies in the vector before transmission bull Example anopheles mosquito is a biological vector for malaria
-Mechanical vector A vector is called mechanical vector if the agent is directly infective to other hosts without having to go through a period of multiplication or development in the vector The vector simply carries the agent by its body parts( leg proboscis etc) to convey it to susceptible hosts Example Flies are mechanical vectors for the transmission of trachoma
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Indirect transmission
bull Airborne which may occur by dust or droplet nuclei (dried residue of aerosols) Example Tuberculosis When pulmonary tuberculosis patients cough they emit many aerosols which consists the agents of tuberculosis When these aerosols dry droplet nuclei will be formed These droplet nuclei will remain suspended in the air for some time When another healthy susceptible individual breaths heshe will inhale the droplet nuclei and become infected with tuberculosis
bull Examples -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma
-Injury site is portal of entry for tetanus
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
VI Susceptible human host
bull The susceptible human host is the final link in the infectious process Host susceptibility or resistance can be seen at the individual and at the community level
bull Host resistance at the community (population) level is called herd immunity
bull Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent based on the immunity of a high proportion of individual members of the population thereby lessening the likelihood of a person with a disease coming into contact with b susceptible
bull Example - If 90 of the children are vaccinated for measles the remaining 10 of the children who are not vaccinated might not become infected with measles because most of the children (90 ) are vaccinated That means transmission from infected person to other susceptible children will not be easier
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull Disease Incidence Percentage of population that contracts a disease in a given time period
bull Disease Prevalence Percentage of population that has the disease during a given time period
Basic Epidemiological Terms
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Levels of disease
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull Sporadic Disease occurs only occasionally (ie Polio in US)
bull Endemic Disease constantly present in population (ie common cold or ear infection)
bull Epidemic Disease Many people acquiring a disease in a short time period (ie Influenza Gonorrhea AIDS)
bull Pandemic Disease Worldwide Epidemics (ie Influenza and AIDS)
Basic Epidemiological Terms
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Epidemic
bull ldquoThe unusual occurrence in a community of disease specific health related behavior or other health related events clearly in excess of expected occurrencerdquo
bull (epi= upon demos= people)
bull Epidemics can occur upon endemic states too
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Endemic
bull It refers to the constant presence of a disease or infectious agent within a given geographic area or population group It is the usual or expected frequency of disease within a population
bull (En = in demos = people)
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Hyperendemic and holoendemic
bull The term ldquohyperendemicrdquo expresses that the disease is constantly present at high incidence andor prevalence rate and affects all age groups equally
bull The term ldquoholoendemicrdquo expresses a high level of infection beginning early in life and affecting most of the child population leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (eg malaria)
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Pandemic and Exotic
bull An epidemic usually affecting a large proportion of the population occurring over a wide geographic area such as a section of a nation the entire nation a continent or the world eg Influenza pandemics
bull Exotic diseases are those which are imported into a country in which they do not otherwise occur as for example rabies in the UK
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Sporadic
bull The word sporadic means ldquoscattered aboutrdquo The cases occur irregularly haphazardly from time to time and generally infrequently
bull The cases are few and separated widely in time and place that they show no or little connection with each other nor a recognizable common source of infection eg polio meningococcal meningitis tetanushellip
bull However a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Attack rates and primarysecondary cases
bull Attack rate proportion of non-immune exposed individuals who become clinically ill
bull Primary (index)secondary cases The person who comes into and infects a population is the primary case Those who subsequently contract the infection are secondary cases Further spread is described as waves or generations
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Zoonosis epizootic and enzootic
bull Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man eg rabies plague bovine tuberculosishellip
bull An epizotic is an outbreak (epidemic) of disease in an animal population eg rift valley fever
bull An Enzotic is an endemic occurring in animals eg bovine TB
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Nosocomial infections
bull Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility
bull It has to be a new disorder unrelated to the patientrsquos primary condition
bull Examples include infection of surgical wounds hepatitis B and urinary tract infections
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease
bull For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull Virulence the severity of disease that the agent causes to the host
bull The Host is the organism that is susceptible to the effect of the agent The status of the host is very important and is generally classifiable as susceptible immune or infected The hostrsquos response can vary from showing no effect to manifesting subclinical disease atypical symptoms straight forward illness or severe illness
Basic Epidemiological Terms
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull The Portal of Exit is a pathway by which the agent can leave the source This pathway is usually related to the pathway where the agent is localized
Basic Epidemiological Terms
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull A Portal of Entry a pathway into the host that gives the agent an access to the tissue where it can multiply or act Often the agent enters the host the same way it left the source
Basic Epidemiological Terms
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
bull The Environment is the conditions or influences that are not part of either the agent or the host but that influence their interaction A wide variety of factors including physical climatologic biologic social and economic factors can come into play
Basic Epidemiological Terms
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Carriers
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
CARRIERS
A Carrier is defined as an infected
person or animal that harbors a
specific infectious agent in the
absence of discernible clinical
disease and serves as a potential source of infection for others
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Carriers
bull It occurs either due to inadequate treatment or immune response the disease agent is not completely eliminated leading to a carrier state
FEATURES OF CARRIER
bull Three elements have to occur to form a carrier state
1 The presence in the body of the disease agent
2 The absence of recognizable symptoms and signs of disease
3 The shedding of disease agent in the discharge or excretions
4 As a source of infection to others
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
CARRIERS
bull TYPE
A) Incubatory
B) Convalescent
C) Healthy
bull DURATION
A)Temporary
B)Chronic
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Opportunistic infection
bull This is infection by organisms that take the opportunity provided by a defect in host defense (eg immunity) to infect the host and thus cause disease For example opportunistic infections are very common in AIDS Organisms include Herpes simplex cytomegalovirus
bull M tuberculosishellip
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Cases
bull A case is defined as ldquoa person in the population or study group identified as having the particular disease health disorder or condition under investigationrdquo
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Virulence and Case Fatality Rate
bull Virulence is the degree of pathogenicity the disease evoking power of a micro-organism in a given host Numerically expressed as the ratio of the number of cases of overt infection to the total number infected as determined by immunoassay When death is the only criterion of severity this is the case fatality rate
bull Case fatality rate for infectious diseases is the proportion of infected individuals who die of the infection This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Serial interval and Infectious period
bull Serial interval (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time)
bull Infectious (communicable) period length of time a person can transmit disease (sheds the infectious agent)
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Incubation and Latent periods
bull Incubation period time from exposure to development of disease In other words the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question
bull Latent period the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period)
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Transmission Probability Ratio (TPR)
TPR is a measure of risk transmission from infected to susceptible individuals during a contact
TPR of differing types of contacts infectious agents
infection routes and strains can be calculated Transmission probabilities bull p00 tp from unvaccinated infective to unvaccinated
susceptible bull p01 tp from vaccinated infective to unvaccinated
susceptible bull p10 tp from unvaccinated infective to vaccinated
susceptible bull p11 tp from vaccinated infective to vaccinated susceptible
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
TPR (cont)
bull To estimate the effect of a vaccine in reducing susceptibility compare the ratio of p10 to p00
bull To estimate the effect of a vaccine in reducing infectiousness compare the ratio of p01 to p00
bull To estimate the combined effect of a vaccine compare the ratio of p11 to p00
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
Eradication and Elimination
bull Termination of all transmission of infection by the extermination of the infectious agent through surveillance and containment
bull Eradication is an absolute process an ldquoall or nonerdquo phenomenon restricted to termination of infection from the whole world
bull The term elimination is sometimes used to describe eradication of a disease from a large geographic region Disease which are amenable to elimination in the meantime are polio measles and diphtheria
And Enjoy
And Enjoy