Keeping healthy

How our bodies defend themselves against infectious diseases

GCSE Science A (B1.1.2)

Candidates should use their skills, knowledgeand understanding to:

■ relate the contribution of Semmelweiss in controlling infection to solving modern problems with the spread of infection in hospitals

■ explain how the treatment of disease has changed as a result of increased understanding of the action of antibiotics and immunity■ evaluate the consequences of mutations of bacteria and viruses in relation to epidemics and pandemics

■ evaluate the advantages and disadvantages of being vaccinated against a particular disease.

a) Microorganisms that cause infectious disease are called pathogens.

b) Bacteria and viruses may reproduce rapidly inside the body and may produce poisons (toxins) that make us feel ill. Viruses damage the cells in which they reproduce.

c) The body has different ways of protecting itself against pathogens.

d) White blood cells help to defend against pathogens by:

■ ingesting pathogens

■ producing antibodies, which destroy particular bacteria or viruses

■ producing antitoxins, which counteract the toxins released by the pathogens.

e) The immune system of the body produces specific antibodies to kill a particular pathogen. This leads to immunity from that pathogen. In some cases, dead or inactivated pathogens stimulate antibody production.If a large proportion of the population is immune to a pathogen, the spread of the pathogen is very much reduced.

f) Semmelweiss recognised the importance of hand-washing in the prevention of spreading some infectious diseases. By insisting that doctors washed their hands before examining patients, he greatlyreduced the number of deaths from infectious diseases in his hospital.

g) Some medicines, including painkillers, help to relievethe symptoms of infectious disease, but do not kill the pathogens.

h) Antibiotics, including penicillin, are medicines that help to cure bacterial disease by killing infectious bacteria inside the body.

Antibiotics cannot be used to kill viral pathogens, which live and reproduce inside cells. It is important that specific bacteria should be treated by specific antibiotics. The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. Overuse and inappropriate use of antibiotics has increased the rate of antibiotic resistant strains of bacteria.

i) Many strains of bacteria, including MRSA, have developed resistance to antibiotics as a result of natural selection. To prevent further resistance arising it is important to avoid overuse of antibiotics.

j) Mutations of pathogens produce new strains.Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen. The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.

k) The development of antibiotic-resistant strains of bacteria necessitates the development of new antibiotics.

Higher Tier candidates should understand that:

■ antibiotics kill individual pathogens of the non-resistant strain■ individual resistant pathogens survive and reproduce, so the population of the resistant strain increases■ now, antibiotics are no longer used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down.

l) People can be immunised against a disease by introducing small quantities of dead or inactive forms of the pathogen into the body (vaccination). Vaccines stimulate the white blood cells to produce antibodies that destroy the pathogens. This makes the person immune to future infections by the microorganism. The body can respond by rapidly making the correct antibody, in the same way as if the person had previously had the disease. MMR vaccine is used to protect children againstmeasles, mumps and rubella.

m) Uncontaminated cultures of microorganisms arerequired for investigating the action of disinfectantsand antibiotics.For this:■ Petri dishes and culture media must be sterilisedbefore use to kill unwanted microorganisms■ inoculating loops used to transfer microorganismsto the media must be sterilised by passing themthrough a flame■ the lid of the Petri dish should be secured withadhesive tape to prevent microorganisms fromthe air contaminating the culture.

n) In school and college laboratories, cultures should be incubated at a maximum temperature of 25°C, which greatly reduces the likelihood of growth of pathogens that might be harmful to humans.

o) In industrial conditions higher temperatures can produce more rapid growth.

GCSE style question

Vaccines like MMR, give people immunity against infectious diseases.

Which three diseases does MMR vaccine protect against?

Explain how vaccinations helps to protect the body against a disease.

The body’s immune system protects us from diseases.Describe the different ways in which white blood cells protect us from infectious diseases.

June 2011

ingest / digest pathogens / bacteria / viruses produce antibodies produce antitoxins any one from: • (antibodies) destroy or kill pathogens / bacteria / viruses / germs • (antitoxins) counteract / destroy /neutralise toxins / poisons • reasonable reference to memory cells or rapid production of antibodies upon re-infection

Influenza is caused by a virus.(a) How do viruses cause illness?

(1 mark)(b) A British company making a reality television show in the Peruvian Amazon has been accused of starting an influenza epidemic. This epidemic allegedly killed fourmembers of a remote Indian tribe and left others seriously ill.The members of the television crew did not show symptoms of influenza, but members of the Indian tribe died from the disease.

Suggest an explanation for this.

Jan 2010

Influenza is an infectious disease. The influenza virus often mutates.This has made the World Health Organisation worried about another influenza pandemic.(a) What is meant by a pandemic?

(1 mark)

(b) Explain why a mutation of the influenza virus might cause another influenza pandemic.

June 2010

The MMR vaccine is used to protect children against measles, mumps and rubella.

(a) Complete the sentences about vaccination.

Vaccines stimulate white blood cells to produce .......................................... .

This makes children ....................................................... to the pathogen.

Jan 2009

In the 1990s, many people thought that the MMR vaccine caused autism in some children. As a result, the Japanese government stopped using the MMR vaccine.The graph gives information about the percentage of children in Japan vaccinated with the MMR vaccine and the number of children who developed autism during the 1990s.

Jan 2009

Jan 2009Describe how the percentage of children vaccinated with the MMR vaccine changed between 1990 and 1995.

Does the data in the graph support a link between MMR vaccination and autism? Why?

Influenza is a disease caused by a virus.Explain why it is difficult to treat diseases caused by viruses.

In some years there are influenza epidemics.The graph shows the death rate in Liverpool during three influenza epidemics.

June 2009

The population of Liverpool in 1951 was approximately 700 000.Calculate the approximate number of deaths from influenza in week 4 of the 1951 epidemic.

In most years, the number of deaths from influenza in Liverpool is very low.Explain, in terms of the influenza virus and the body’s immune system, why there were large numbers of deaths in years such as 1918 and 1951.

In the 19th century, Dr Semmelweiss investigated infection in a hospital.He compared the number of deaths of mothers on two maternity wards.• On Ward 1, babies were delivered mainly by doctors. These doctors worked on many different wards in the hospital.• On Ward 2, babies were delivered by midwives. The midwives did not work on other wards.The bar chart shows the results of his investigations.

June 2009

June 2009

600 mothers gave birth on Ward 2 in 1845.How many mothers died from infections on Ward 2 in 1845?

Which was the safer ward on which to have a baby?

In January 1848, Dr Semmelweiss asked all doctors to wash their hands before delivering babies.The table shows the number of deaths on the two wards in 1848.

What was the effect on the death rate on Ward 1 of doctors washing their hands before delivering babies? What might the reason be?

Explain, as fully as you can, how MRSA strains of bacteria became difficult to treat.

Jan 2011