Now, plainly, most of the people stricken with Mickey Mouse measles do notunderstand how vaccines work, because they didn't get them. The vast majority of the infected were unvaccinated against the disease, including kids who were too young for the shots and anti-vaxxers who chose against them. That's how you get an outbreak. But six of the cases got their measles-mumps-rubella vaccine—the MMR shot—and still managed to get infected. And all but two of them had gotten at least two doses, the standard recommendation.

So what happened?

The measles vaccine is actually one of the most effective vaccines in the world. According to Greg Wallace, lead of the measles, mumps, rubella and polio team at the CDC, two doses are 97 percent effective against infection. (Compare that to 88 percent for two doses of the mumps vaccine from the MMR shot.) It's a live version of the virus, just weakened—or attenuated—so it doesn't cause severe symptoms. The vaccine replicates just like the full-on measles virus, inciting your immune system to produce antibodies against it. Those antibodies then protect against actual measles as well.

But in some people, that response just doesn't happen. No one knows why. Either your body doesn't produce enough antibodies, or the ones it does produce aren't specific enough to latch on to the virus and kill it.

That's why the CDC recommends two doses of the vaccine: After the first dose, 5 to 7 percent of people won't have a good enough antibody response to protect them. A second dose ensures that enough people get antibodies above that protective threshold to control the disease. "And even with two doses, you can get some failure," says Wallace, "whether it's because the initial response isn't perfect, or because the response waned in some people.