10.1.6Diseases and Immunity
Preventing the Spread of Disease — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 7
Part 1 of 5Vaccination protects the individual
How a vaccine helps
- How a vaccine helps A vaccine contains a harmless form of a pathogen,
- such as a weakened or dead microbe or part of its surface,
- which stimulates the immune system to make antibodies and memory cells.
- If the real pathogen later enters the body, the immune response is fast enough to destroy it before symptoms develop.
Immune people do not spread disease
- Immune people do not spread disease A person who is immune because of vaccination cannot become infected,
- so they cannot pass the pathogen on to others.
- Every immune person is therefore a barrier that breaks a chain of transmission.
Limits of vaccination
- Limits of vaccination Vaccines exist only for some diseases;
- there is no effective vaccine for every pathogen.
- Some pathogens change their surface antigens quickly, so a vaccine may need to be updated regularly.
Part 2 of 5Herd immunity (HT only)
1High vaccination rate
- High vaccination rate A large proportion of the population is immune to the pathogen.
2Few places to reproduce
- Few places to reproduce The pathogen can only multiply inside the bodies of the unvaccinated minority, so it has very few hosts.
3Rare contact
- Rare contact An unvaccinated person is unlikely to meet an infected person, so the disease passes on very slowly or not at all.
4Whole population protected
- Whole population protected Even people who are not vaccinated gain protection. This is called herd immunity.
Part 3 of 5When vaccination levels fall (HT only)
1
Fewer people in the population are vaccinated, so the proportion of susceptible people grows.
2
An infected person is now much more likely to come into contact with someone who can catch the disease.
3
The number of infections rises and the pathogen spreads quickly through the community.
4
More people become seriously ill and the number of deaths from the infectious disease increases.
5
In the worst case the disease spreads across a country as an epidemic or across the world as a pandemic.
Part 4 of 5Control or eradication (HT only)
Why children are vaccinated
- Why children are vaccinated Children are seen regularly by doctors and nurses,
- so they can be vaccinated early and the level of immunity in the population stays high.
- Keeping the vaccinated proportion high is what prevents epidemics and pandemics.
Eradication programmes
- Eradication programmes Some vaccination programmes aim to remove a dangerous disease completely instead of just keeping cases low.
- Smallpox is the example: after a worldwide programme run by the World Health Organisation,
- which began in the 1950s, it was declared eradicated in 1980.
Part 5 of 5Comparing the ideas
| Situation | What happens | Result |
|---|---|---|
| Person vaccinated | Memory cells and antibodies are ready | Individual cannot catch or spread the disease |
| High percentage vaccinated (HT only) | Pathogen has few hosts | Herd immunity protects everyone |
| Vaccination rate drops (HT only) | More susceptible people meet infected people | Rise in infections and deaths |
| Global programme succeeds (HT only) | Pathogen has nowhere left to survive | Disease eradicated, as with smallpox |
Key terms
| Term | Meaning |
|---|---|
| Vaccination | Giving a person a harmless form of a pathogen so that they become immune without suffering the disease. |
| Immune | Protected from a disease because the body can quickly make antibodies against that pathogen. |
| Herd immunity | Protection of a whole population when enough people are immune that the pathogen cannot spread easily. |
| Epidemic | A large outbreak of a disease in one region or country. |
| Pandemic | An outbreak of a disease that spreads across many countries or the whole world. |
| Eradication | Complete removal of a disease from the world so that it no longer occurs. |
Avoid these mistakes
1
Saying that a vaccine contains medicine that kills the pathogen in the body — it trains the immune system; it does not treat an existing infection.
2
Thinking that a vaccine exists for every disease — many pathogens have no vaccine.
3
Stating that herd immunity means everyone is vaccinated — it only needs a large enough percentage, and it also protects those who are not vaccinated.
4
Forgetting to say that the pathogen can only reproduce in unvaccinated people when explaining herd immunity.
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