Infectious DiseaseBMTMrs. Cavanaugh

• Disease• Condition that impairs

normal tissue function. Examples include:

• Cystic Fibrosis• Atherosclerosis• measles

• Infectious Disease• Caused by the invasion

of a host by agents whose activities harm the host’s tissues (cause disease) and can be transmitted to other individuals (they are infectious)

Microorganisms• Capable of causing disease and are called pathogens.• Usually disease causing m/o’s get the most attention, however,

it’s important to note that most m/o’s do NOT cause disease.• Many provide protection.

• Opportunistic Pathogen• Potentially infectious

agents that rarely cause disease in individuals with healthy immune systems.

• Typically found among groups such as • Elderly (immune

systems are failing)• Cancer patients

(chemotherapy and radiation) which affect immune systems

• HIV positive or AIDS (altered immune system)

• True Pathogen• Infectious agent that

causes disease in virtually any susceptible host.

Two terms not synonymous• Infection• Results when a

pathogen invades and begins growing within a host.

• Disease• Results only if and

when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired.

• Our bodies have defense mechanisms to prevent infection. If the defense mechanisms fail, we have the ability to prevent disease after infection occurs.

• Infectious agents that are easily transmitted are considered contagious.

• Infectious agents that easily cause disease are said to be virulent.

• The most worrisome infectious agents are those that are both very contagious and very virulent.

For disease to occur, pathogens must

• Enter the host body• Adhere to specific

host cells• Invade and colonize

host tissues

• Inflict damage on those tissues

• Pathogen typically enters the host through natural orifices such a the mouth, eyes, or genital openings

• Or through wounds that breach the skin barrier to pathogens

• Some pathogens can grow at the initial entry site, however most must invade areas of the body where they are not typically found to grow.

6 major types of infectious agents

• Bacteria• Viruses• Fungi• Protozoa• Helminths

• Prions – most recently recognized

Bacteria• Unicellular prokaryotic organisms• They have no organized internal membranous structures such as

nuclei, mitochondria or lysosomes• Most reproduce by growing and dividing into two cells• Much diversity among bacteria

• Most common are• Bacillus (rod-shaped)• Coccus (spherical)• Spirillum (helical rods)

-Some bacteria obtain energy from the sun.- Others oxidize inorganic compounds to supply their energy

needs.-Still other bacteria generate energy by breaking down organic compounds such as amino acids and sugars.

2 broad classes of bacteria• Gram-negative

bacteria• Appear pink after a

staining process• Example – Samonella

typhi which causes Typhoid Fever

• Yersini pestis which causes plague

• Gram-positive bacteria• Appear purple after

staining process• Example – Staphyloccus

aureus which cause skin, respiratory and wound infections

• Clostridium tetani which produces a toxin that could be lethal

Viruses• Can infect all

organisms• Including plants,

animals, fungi and bacteria

• Cause disease like other infectious agents by disrupting normal cell function.

• Viruses are not organisms themselves.• They have no

metabolism and can not reproduce without a host cell.

Classified according to shape, size and type of genome• DNA viruses• herpes viruses that

cause chicken pox, cold sores and genital lesions• Poxvirus that cause

smallpox

• RNA viruses• Rhinovirus that cause

most common cold• Myxovirus and

paramyxovirus that cause influenza, measles and mumps

• Rotavirus that cause gastroenteritis

• Retrovirus that cause AIDS and several types of cancers

Fungi• Most are multicellular• Yeast is type of fungus that is unicellular• Also infect plants and animals• Example of dz caused by fungi

• Ringworm• Histoplasmosis (lung infection transmitted by bat or bird droppings)

Yeast• Type of fungi (Candida genus)• Opportunistic pathogen• Causes thrush in mouth and throat and common vaginal

infections in people who are immunocompromised or undergoing antibodic therapy• Antibiotics reduce the bacterial population normally present in the

throat and vagina, allowing yeast to grow

Protozoa• Unicellular • Amoeba and paramecium• No cell wall making them capable of rapid and flexible

movements• Acquired through contaminated food or water or by the bite

of infected anthropod such as a mosquito

• Diarrheal disease in the US caused by• Giardia lamblia• Cryptosporidium

parvum

• Malaria• Tropical illness that

causes 300-500 million cases annualy, caused by Plasmodium

Helminths• Complex reproductive cycles with multiple stages, many or all

of which require a host• Schistosoma

• Flatworm that causes swimmers itch• Trichinella spiralis

• Roundworm - ingested in improperly cooked pork from infected pigs.• Early sx incude vomiting, diarrhea, and fever• Later sx include intense muscle pain as larvae grow and mature in the

tissues• Fatal cases often show congestive heart failure and respiratory paralysis.

Prions• Evidence has linked degenerative disorders of the central

nervous system to infectious particles that are made up only of protein.• Creutzfeldt-Jakob disease (humans)• Scrapie (sheep)• Bovine spongiform encephalopathy – also known as “mad cow

disease” (cattle)

• All known prion diseases frequently result in brain tissue that is riddled with holes.

• Some are inherited• Others are apparently due to infection by eating infecected

tissue or inadvertently through medical procedures such as tissue transplants.

Creuzfeldt-Jakob disease

• A rare, degenerative, invariably fatal brain disorder; believe caused by an unusual "slow virus" or another organism • Typically, onset of symptoms occurs

about age 60, and about 90 percent of individuals die within 1 year. • Characterized by rapidly progressive

dementia and they eventually lose the ability to move and speak and enter a coma

Occurrence of Infectious Disease• Epidemiology• The study of the occurrence of disease in populations

• Epidemiologist (specialist in this area) – work to prevent or minimize the impact of diseases in the population.• Work may include activities of identifying unusually high

incidences of disease, determining the effectiveness of a vaccine and calculating the cost effectiveness of various means of controlling disease transmission.

• Concern is with infectious and non-infectious disease• Non infectious includes cancer, atherosclerosis, lead poisoning

Disease Reservoirs• The site where the infectious agent survives

• Humans• Measles virus because it does not infect other organisms

• Animals• For diseases that infect humans

• Ex – bacteria that causes plague (Yersinia pestis) • Rodents are the reservoir

• Soil• Nonliving reservoir

• Fungi and bacteria• Clostridium tetani – Causes Tetanus

Mode of Transmission• Direct Contact• Contact with the

reservoir• Ex – touching infected

person, ingesting infected meat, or being bitten by infected animal

Also includes inhaling the infectious agent in droplets emitted by sneezing or coughing.

• Indirect Contact• Occurs when

pathogens can withstand the environment outside its host for a long period of time before infecting another individual• Ex- from toys, door

knobs, contaminated water

• Horizontal Transmission• Agent is passed from

person to person in a group

• Includes cholera and rotovirus

• Vertical Transmission• Transmitted from

parent to child during reproduction, fetal development or birth• Includes AIDS, Herpes

encephalitis

Host defenses against infectious disease• Nonspecific mechanisms include the body’s primary defense

against disease• Include anatomical barriers• Ex – skin, nasal openings, skull and vertebral column

• Difficult to treat burn patient with tissue breakdown

• Physiological deterrents• Tears, vaginal secretions, saliva and nasal secretions, blood, sweat• Inflammatory response

• Normal flora• Microorganisms present on and in the body normally.• Typically do not cause disease in the body• Protect the host by competing with invading harmful bacteria

Breakdown in Normal Flora• Growth of normal flora is depressed during altered immunity• Sickness, antibiotic treatment, disease

• Opportunistic infections can occur at this time

Immunity• When a host encounters an antigen that it recognizes.• Memory cells are present and begin to respond by producing

antibodies to fight off the invading pathogen.• Usually a very rapid response.

• Vaccinations cause this memory response and typically prevents first occurrences of many diseases.

Vaccination• Either a killed or weakened (attenuated) strain of a particular

pathogen, or a solution containing critical antigens from the pathogen.

• Body’s immune system will respond to these vaccines

• As a result, memory lymphocytes will be present that respond rapidly if the actual pathogen is encountered.

• The resulting rapid activation of immune cells prevents disease.