Health officials fear thousands may have been exposed to the measles at Disneyland and Disney California Adventure last month.

The number of those infected keeps growing, and as Linh Bui reports, most of those who got the disease were not vaccinated against it.

Of the 20 people infected by the current outbreak, at least 15 were not vacated.

With tens of thousands of visitors from around the world in close contact everyday, the “happiest place on earth” was an ideal place for the highly contagious disease to spread.

“You can be reasonably near someone, or breathe it in overtime and you can become infected,” said Doctor Mark Zahn.

Measles infections in the U.S. skyrocketed last year, with 610 cases reported. That’s the highest number since 2000, when the disease that causes a rash, high fever, and red, watery eyes, was considered to be eliminated.

The CDC says the increase is tied to a decline in child vaccinations.

“The biggest concern is you’re gonna have the potential for more outbreaks,” said Dr. Zahn.

Disneyland is located in Orange County, California, which is reported the highest rate of measles in the state last year. It ‘s also the home to some state’s highest numbers of unvaccinated children.

Orange County pediatrician doctor Bob Sears is part of a small minority in the medical community, that tells parents vaccinations are not always necessary.

“You have to discuss the pros and the cons and most doctors only discuss the pros,” said Sears.

The CDC says the risk of a serious allergic reaction to vaccines is less than a million still. Doctor Sears sayss half of his patients choose not to vaccinate.

“Parents just don’t fear the diseases anymore and parents also don’t want their child to have a bad side effect,” said Sears.

Measles cases have now been reported in California, Utah, Colorado, and Washington.

Disneyland released a statement when the outbreak was reported saying it is working with the California Department of Health in its investigation.

The park has not made a statement about the increase in infections.