A measles outbreak that started at Disneyland in California is spreading further across the country and into Mexico as people who were on their vacations take the virus home to unvaccinated family members.

More than four dozen people in four states have been diagnosed with measles related to Disneyland or the adjacent Disney California Adventure Park in Orange County.

The first group of patients to develop measles had visited the parks between Dec. 15 and Dec. 20.  People can develop measles three to 21 days after being exposed.

At least 42 cases of measles related to Disney have been diagnosed in California, plus three in Utah, two in Washington, one in Colorado and one in Mexico.

The outbreak has the potential to develop into one of the worst outbreaks since 1989.