There's a measles outbreak at Disneyland. Here's what you need to know.

1. An ongoing measles outbreak that started at Disneyland in Orange County, California has infected 67 people and alarmed public health officials.
2. Most of those infected were not properly immunized against the disease, including six infants who were too young to be vaccinated.
3. The outbreak is part of an uptick in measles in the United States, with more cases reported last year than any other year in the past two decades.

Measles is extremely infectious

Measles is a deadly, infectious disease that typically strikes children. The disease comes on as a fever and runny nose, and causes an uncomfortable blotchy, rash all over the body. It's airborne, so it spreads quite easily, too: it just takes an infected person breathing or coughing near someone who is unvaccinated. In an unimmunized population, one person with measles can infect 12 to 18 others.

For these reasons, the disease quickly and easily moved through Disneyland in Orange County and beyond. Though "patient zero" in this case hasn't yet been identified, the fact that he or she was surrounded by people who weren't vaccinated helped the disease to reach 70 people in nine California counties, as well as Utah, Washington, Oregon, Colorado, and Mexico. The California department of health found that 42 of these confirmed cases could be traced back to the Disney parks in Anaheim.

A quarter of the patients in this outbreak have been hospitalized, according to USA Today, and more than 80 percent were not vaccinated against the disease, including six infants too young to be vaccinated (since the measles vaccine is not licensed for use on babies younger than 12 months).

Five people who contracted measles during this outbreak had been vaccinated. Health officials told the LA Times that this is explained by the fact that there's a five percent risk of vaccine failure in people who got their vaccinations at a time when only one dose was common (instead of the current recommendation of two). With a double dose of the MMR vaccine, the failure rate falls to less than one percent.

The anti-vaccine movement may be contributing to the uptick of cases

Pediatric infectious disease specialist James Cherry told the New York Times the outbreak was "100 percent connected" to the anti-vaccine movement. "It wouldn’t have happened otherwise — it wouldn’t have gone anywhere," he said.

The state has been a hotbed of vaccine denialism, the Times reported:

The vaccination exemption rate among kindergarten students in California — cases in which parents said they did not want their children vaccinated for health, religious or other reasons — was 3.1 percent in the 2013-14 school year, according to the C.D.C. report. Oregon had an exemption rate of 7.1 percent, the nation’s highest, the report found. Health officials said the vaccination rate needed to be above 95 percent in all communities to prevent outbreaks.

Still, the California figure can be deceiving. Health officials said there were pockets across the state, including wealthy neighborhoods in Los Angeles and Orange Counties and enclaves in Northern California, where the exemption rate jumped into the double digits. California has long been viewed as particularly prone to this kind of outbreak because of its population size and the number of people arriving from overseas.

Measles is indeed an entirely preventable disease, and was declared eliminated in the US in 2000. Again, two doses of of the MMR vaccine — which has been available here since 1963 — are considered safe and more than 99 percent effective at preventing measles.

But global travel from countries where the disease is circulating may also be sparking the rise in cases here

The disease seems to be making a comeback in the US. Last year, the New England Journal of Medicine reported that there were more measles cases of measles in 2014 than during any year in the past two decades.

New England Journal of Medicine

(Chart courtesy of the New England Journal of Medicine)

The New England Journal author wrote that, in addition to vaccine refusal, there is another key reason measles is spreading in the US: the disease hasn't been eliminated everywhere, and it seems travelers to America are bringing measles with them.

In an examination of 2014 measles outbreaks, the Centers for Disease Control and Prevention reported, "Of the 288 cases, 280 (97 percent) were associated with importations from at least 18 countries."

Many of these travelers were coming back from the Philippines, which has been dealing with a massive outbreak since fall 2013.

These travelers would not be getting sick, however, if they were vaccinated. According to the CDC, of the cases examined in 2014, 195 involved US residents who were unvaccinated. Eighty-five percent of these people had refused vaccination because of religious or personal beliefs.

So Disneyland may have been the perfect incubator for a measles outbreak, with its mixture of international travelers and very young unimmunized children, in a state where vaccine refusal is not uncommon.

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