Anti-vaccination beliefs are contagious like a disease, argues Malia Jones and Alison Buttenheim in the Washington Post:

How do these clusters of exempted kids form? We think that the idea of vaccines being unsafe or ineffective or unnecessary spreads—like a disease itself—from person to person within social networks.  Parents talk to their friends, other parents at their child’s school, and to their neighbors.  Normal social processes produce clusters of vaccine refusers in the social landscape.  When it’s time to enroll in school, like-minded parents are drawn to certain schools—and this is especially pronounced when parents have the resources to be choosy about their child’s education.  Soon, there are schools and communities where vaccine refusal is not an anomaly; it is the norm.