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Following an immunisation, a person can become immune to the specific disease. This immunity gives protection against illness in that individual. When the majority of the population have immunity against a serious disease, this means that even those people who are not immune will still be protected because they are less likely to come into contact with an infected person. This type of immunity is herd immunity.

There are three recognised scenarios in relation to herd immunity, described below.

1. The majority of the population are not immune to a specific disease, however, a few people are ill and contagious. This can develop easily into a mass infection because the majority of the population aren't immune.
2. Most of the population are not immune to the specific disease but are well, some are immune and healthy, and a few are not immune, but ill and contagious. Mass infection can result again, but a small number of immune individuals remain healthy and some that are not immune will also be healthy.
3. The majority of the population are immune and healthy against a specific disease, a few are not immune but well. A few are not immune, and they are ill and contagious. The result is that the majority are protected due to the high level of immunity. A few individuals will still become ill, but the large number of immune individuals gives protection.

If the number of people immune to a specific disease drops in a population, it leaves the rest of the population at risk of mass infection, as they are more likely to come across people who are infected and contagious. This increases the number of infections, as well as the number of people who could die from a specific infectious disease.