Vaccination

A vaccine contains small quantities of a dead or inactive form of a pathogen. It is usually injected, though some are taken by mouth or as a nasal spray.

Because the pathogen is dead or inactive, it can't cause the disease — but it still carries the pathogen's antigens.

1. The vaccine introduces antigens from a dead or inactive pathogen into the body.
2. White blood cells recognise the antigens and produce antibodies against them.
3. Memory cells are also produced.
4. If the live pathogen later enters the body, memory cells cause the right antibodies to be produced quickly and in large amounts.
5. The pathogen is destroyed before it can cause illness — the person is immune.

If a large proportion of a population is vaccinated, the pathogen has few people it can infect, so its spread is greatly reduced. This protects people who can't be vaccinated — such as newborn babies or people with weakened immune systems.

This is called herd immunity.

Pros:

• Has controlled many serious diseases, such as measles and polio; smallpox has been wiped out entirely.
• Prevents epidemics by reducing how far a disease can spread.

Cons:

• Vaccines don't always give immunity.
• Some people have side effects, such as a sore arm or mild fever; serious reactions are very rare.