NHS direct again, there is a funny little video clip illustating it as well
What is herd immunity?
If enough people in a community are immunised against certain diseases, then it is more difficult for that disease to get passed between those who aren't immunised.
Herd immunity does not apply to all diseases because they are not all passed on from person to person. For example, tetanus can only be caught from spores in the ground.
Herd immunity can also be affected by:
- environmental factors
- strength of people's individual immune systems
- how infectious the disease is.
Environmental factors
People living in crowded conditions, such as inner cities, are more at risk of catching some diseases than people living in the country because of close contact with other people.
There are also seasonal increases in diseases such as meningitis and flu during the winter months.
Strength of a person's immune system
People whose immune systems are not working well, either because they have a disease or because of treatment for it, may not be able to have the immunisations. They are therefore at greater risk of catching the diseases for which we immunise.
How infectious the disease is
As an example, measles is highly infectious. If everyone stopped being immunised against it, then everyone would catch it. We know that we need at least 90% of children to be immune to stop the disease being spread.
If 95% of children are protected by MMR, then we can eliminate not just measles, but mumps and rubella as well. In Finland, this has already been achieved.