Herd immunity

If enough members of a group of animals (including humans) are immunized against a disease, the entire group is more likely to escape infection.

The problem with herd immunity, however, is deciding who has to get the shots. Individually healthy people would prefer not to go through the effort of getting the shot and “free ride” on those who do. The situation is akin to the prisoner’s dilemma, which may explain the low vaccination rate in the U.S.