Melinda Gates of the Bill and Melinda Gates Foundation says that vaccine denial is a privilege that those in developing countries, which see the daily toll of diseases like measles, simply don't have.

"We take vaccines so for granted in the United States," Gates told the Huffington Post in a prerecorded interview aired on January 22. "Women in the developing world know the power of [vaccines]. They will walk 10 kilometers in the heat with their child and line up to get a vaccine, because they have seen death. [Americans have] forgotten what measles deaths look like."

She added, "I'd say to the people of the United States: we're incredibly lucky to have that technology and we ought to take full advantage of it."

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In response to the Disneyland outbreak in January 2015, 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 key is what the scientific community calls herd or community immunity. If every American of age was vaccinated, measles wouldn't spread much further even if foreign travelers came into the country with the disease — as appears to be the case with measles. Vaccinated people essentially act as barriers to measles outbreaks, since the disease can't pass through them and infect other people. The awful truth of the anti-vaccine movement is that it puts the most vulnerable populations at risk: infants under 12 months of age, who can't get vaccinated and are more susceptible to infection, and the elderly, who have a higher risk of death if they contract these illnesses.