Health officials fear thousands may have been exposed to the measles at Disneyland and Disney California Adventure last month. Most of those who got the disease were not vaccinated against it.

With tens of thousands of visitors from around the world -- in close contact -- every day, the "Happiest Place on Earth" was an ideal place for the highly contagious disease to spread, according to epidemiolgist, Dr. Mark Zahn.

"You can be reasonably near someone, or breathe it in over time and you can become infected," said Zahn.

A measles outbreak is being blamed on a lack of vaccinations in Orange County, Calif. CBS News

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 have been 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 Zahn.

Disneyland is located in Orange County, Calif., which reported the highest rate of measles in the state last year. It's also home to some of the state's highest numbers of unvaccinated children. Of the 20 people infected by the current outbreak, at least 15 were not vaccinated.

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

Although a doctors responsibility may be to give parents the best medical science can offer, Spears says "you have to discuss the pros and the cons and most doctors only discuss the pros."

The CDC says the risk of a serious allergic reaction to vaccines is less than one in a million. Still, when CBS News spoke to Dr. Sears last summer, he said 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," he said.

Rebecca Estepp stopped vaccinating her children after her son had a severe reaction. She encourages other parents to think twice.

"I think what parents are wanting are choices," said Estepp. "They know they are taking this risk and that their child may be susceptible to some of the infectious diseases out there."

Most doctors say the Disneyland outbreak should be a wake-up call.