Disneyland is now offering to test all employees for measles and if needed, provide them with the vaccine.

A Disney employee, who asked not to be identified but says knowing her co-workers have been infected is reason to worry.

"It is scary thinking that the happiest place on earth is infected with measles now," she said. "All these kids that don't get vaccinated, they could easily, you know, come meet princesses, or characters, and come up and hug us."

Measles is highly contagious. One person with measles will infect nine out of ten people around them who have not been vaccinated. Measles had been largely eradicated in the U.S. by the year 2000. But it's back in part because a growing number of parents are not vaccinating their kids, fearing side effects.

"These vaccines have been used for decades, and have been proven to be exceptionally safe," said Dr. Deborah Lehman is an epidemiologist at Cedars Sinai Medical Center in Los Angeles.

There are some parents who believe there's more risk to the vaccine than there is to the measles. But to them Lehman says, "they're wrong and they're putting their own children's lives at risk. One in a thousand children who contract measles will die."

Most children born after 1971 received a measles, mumps and rubella vaccine. It requires two doses to be 99% effective. Doctors say those born before 1957 likely have been exposed to the virus and are immune. Those born between 1957 and 1971 when vaccines weren't as reliable should check with their doctors to see if they've been properly vaccinated.

The Centers for Disease Control says this outbreak is a good reminder for everyone to make sure they received two doses of the measles vaccine. A simple blood test can check your immunity level. As an adult you can get the full vaccination in about a month.