LOS ANGELES -- 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 travelled 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 close-knit religious communities.

The Disneyland outbreak has already spread well beyond the theme parks that attract tens of thousands of visitors from around the globe, who could then return home with the virus. Disease investigators for weeks raced to identify measles-stricken patients, track down potential contacts and quarantine them if necessary.

Public health experts say success at containing the outbreak will largely depend on how many unvaccinated people get the measles shot.

The California outbreak probably began when an infected person spread the illness to a handful of mostly unvaccinated people, who then exposed many others.

In contrast, the Ohio outbreak "stayed contained within those communities, and outside people said, 'Well, it doesn't really affect me.' What's different with this one is more people can relate to Disneyland," said Dr. Gregory Wallace of the Centers for Disease Control and Prevention.

More than 100 measles cases in half a dozen states have been linked to people who visited or worked at Disneyland in December or exposure to infected people who went there. California health officials on Wednesday reported 99 measles cases including six new infections with a Disneyland connection.

Federal health officials said it's too early to predict whether this will be a particularly severe year compared with 2014, which saw more measles cases than any year since 1994. The Ohio outbreak accounted for more than half of the 644 measles cases reported last year.

Homegrown measles has not occurred in the United States since 2000 due to an aggressive vaccination campaign. But outbreaks have hit in recent years with nearly all cases linked to travellers who caught the virus overseas where measles still rages and spread it in this country among pockets of unvaccinated people.

In the Disneyland outbreak, public health officials have yet to identify the "index case" -- the first person who contracted measles and spread it. But they believe it's someone who imported the virus from abroad and spread it during a visit to one of the theme parks days before Christmas.

Since the Disneyland measles outbreak, parents who refuse vaccines for their children on religious or philosophical grounds have been on the defensive against a tide of doctors and public-health officials urging people to get vacinated.

The issue has stirred sometimes-angry debates and even entered the realm of presidential politics.