Herd immunity refers to the level of resistance against a specific communicable disease in the community as a whole. When a high proportion of a community is immune to a particular disease that spreads from person to person (e.g. measles), the infectious agents causing that disease find it difficult to infect any non-immune (susceptible) people. This could result in the infection "dying out" in that community because there are not enough infected people to act as a reservoir for the infectious agents. A high level of herd immunity benefits everyone because it makes it more difficult for a particular infection to spread from person to person through that community.

Two ways in which the level of herd immunity can be increased in a community:

• If a vaccine exists, immunisation of a large proportion of community members is the best way to increase their herd immunity.
• If there is no vaccine, but a large proportion has suffered from a particular infection in the past and recovered from it, herd immunity increases because many people have naturally acquired active immunity.

There are various reasons herd immunity created by vaccination may not be achieved:

• More than one strain of an organism that causes the disease which may not be included in the vaccine.
• Humans may not be the only reservoir for the disease. The virus/bacteria may be found in other animals.
• The virus/bacteria can mutate and the vaccine may not contain the mutated strain.