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