The 189 cases of measles in the U.S. last year is small compared with the 530,000 cases the country used to see on average each year in the 20th century. But, the disease — which started to wane when a vaccine was introduced in 1967 — is one of the most contagious in the world and could quickly go from sporadic nuisance to widespread killer.

Measles kills about once in every 1,000 cases. As cases mount, so does the risk. “We really don’t want a child to die from measles, but it’s almost inevitable,” says Schuchat. “Major resurgences of diseases can sneak up on us.”

Vaccination rates against most diseases are about 90%. Fewer than 1% of Americans forgo all vaccinations, Schuchat says. Even so, in some states the anti-vaccine movement, aided by religious and philosophical state exemptions, is growing, says Paul Offit, chief of infectious diseases at Children’s Hospital of Philadelphia. He points to states like Idaho, Illinois, Michigan, Oregon and Vermont — where more than 4.5% of kindergartners last year were unvaccinated for non-medical reasons — as examples of potential hot spots. Such states’ rates are four times the national average and illustrate a trend among select groups.

“People assume this will never happen to them until it happens to them,” Offit says. “It’s a shame that’s the way we have to learn the lesson. There’s a human price for that lesson.”