Failing to vaccinate your child against measles because it’s against your beliefs isn’t a crime, but perhaps it should be.

This year’s measles outbreak, the nation’s worst in 25 years, stands at 839 reported cases in 23 states as of Friday. The summer months could provide some respite because children are out of school, but many medical professionals expect the situation to worsen before it improves. Last week brought 75 new cases, according to the Centers for Disease Control and Prevention, which is more than the previous...