The largest U.S. measles outbreak in recent history isn't the one that started in December at Disneyland.

It happened months earlier in Ohio's Amish country, where 383 people fell ill after unvaccinated Amish missionaries traveled to the Philippines and returned with the virus.

The Ohio episode drew far less attention, even though the number of cases was almost four times that of the Southern California outbreak, because it seemed to pose little threat outside insolated religious communities.

The Disneyland outbreak has already spread well beyond the theme parks that attract tens of thousands of visitors from around the globe.

Dr. James Cherry is a pediatric infectious disease expert at the University of California, Los Angeles. He says the outbreak "could continue to smolder" if not enough people get vaccinated.