Community Immunity ("Herd" Immunity)
When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease. This is known as "community (or 'herd') immunity." The principle of community immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease.
The top box depicts a community in which no one is immunized and an outbreak occurs. In the middle box, some of the population is immunized but not enough to confer community immunity. In the bottom box, a critical portion of the population is immunized, protecting most community members.