This winter in the San Francisco Bay Area, many children will sit in classrooms and play on the jungle gyms at recess and then go home to attentive parents who work hard to give them every advantage in life. Parents in this part of the country are better educated and wealthier than the average American and can give their children more opportunity. But the Bay Area is also a hotbed of the growing movement to abstain from vaccinations for fear that the shots cause autism and other disorders. Although these parents may have the best of intentions—to protect their kids—they are dangerously misguided.

California is now in the middle of the worst outbreak of pertussis in half a century. This highly contagious disease—known as whooping cough for the distinctive sound its victims make when gasping for air after a fit of paroxysmal coughing—was a scourge of childhood until the advent of an effective vaccine against it in the 1940s, which drastically reduced incidence of the disease. The number of annual cases has been climbing in recent years. Last year, though, the rate of infection rose, once again, to epidemic proportions—7,297 known and suspected cases, a fourfold increase from 2009. Whether those refusing the vaccine have helped fuel the current pertussis epidemic is uncertain, but their decisions have created a public health tinderbox: in some Bay Area schools, 40 percent or more of the kids are not vaccinated, leaving them unprotected against pertussis and other preventable diseases, such as measles.

California is hardly the only state grappling with antivaccine sentiment. Significant numbers of parents across the country are declining standard immunizations for their children. The success of any given vaccine depends on so-called herd immunity, in which a high rate of immunization in a population helps to protect those individuals who are not immune. Herd immunity requires high immunization rates—around 95 percent for highly contagious infections like pertussis and measles. When immunization rates drop below the critical level, disease can strike not only unvaccinated individuals but also vaccinated ones, because all vaccines fail to confer immunity in a certain percentage of people. Parents who opt out are endangering not only their own kids but everybody else’s, too—including those who cannot be vaccinated because they are too young or immunocompromised, as well as youngsters who have received their shots.