Herd immunity
![The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue); the illness spreads freely through the population. The middle box shows a population where a small number have been immunized (shown in yellow); those not immunized become infected while those immunized do not. In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to unimmunized people.](/Images/godic/202501/05/Community_Immunity3303.jpg")
![Charlotte Cleverley-Bisman, who had all four limbs partially amputated at the age of seven months due to meningococcal disease. Mass vaccination can prevent children who are too young to be vaccinated from catching transmissible diseases.[13]](/Images/godic/202501/05/Charlotte_Cleverley-Bisman3303.jpg")
![A cow with rinderpest in the](/Images/godic/202501/05/Rinderpest_milk_fever3303.jpg")
![Measles vaccine coverage and reported measles cases in Eastern Mediterranean countries. As coverage increased, the number of cases decreased.](/Images/godic/202501/05/Measles_cases_coverage_eastern_mediterranean3303.jpg")
Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune. In a population in which a large number of individuals are immune, chains of infection are likely to be disrupted, which stops or slows the spread of disease. The greater the proportion of individuals in a community who are immune, the smaller the probability that those who are not immune will come into contact with an infectious individual.