The varicella zoster virus causes chickenpox in children, then lurks in the body for years and can cause painful shingles later in life. James Cavallini/Science Source hide caption

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A family receives treatment for cholera at a clinic run by Doctors Without Borders in Port-au-Prince, Haiti, in October 2011, a year after the overwhelming outbreak began. Ramon Espinosa/AP hide caption

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Most of the people who got measles in last year's outbreaks hadn't been vaccinated with the MMR vaccine. Photo illustration by Justin Sullivan/Getty Images hide caption

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Parents and teachers who oppose efforts to end the personal belief exemption on vaccinations rally in Los Angeles on April 14. Irfan Khan/LA Times via Getty Images hide caption

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Parents rally against SB 277, a California measure requiring schoolchildren to get vaccinated, outside the Capitol in Sacramento, Calif. Rich Pedroncelli/AP hide caption

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A security fence surrounds the main part of the U.S. Army's Dugway Proving Ground, a testing laboratory in the Utah desert. The Army says it mistakenly shipped live anthrax from Dugway to several labs in the U.S. and Korea. George Frey/Getty Images hide caption

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Worth a little pain? Back in 1990, a school boy got a measles shot in the U.K., and it turns out, he got more than protection against the measles. Photofusion/UIG via Getty Images hide caption

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Australian Prime Minister Tony Abbott looks during a news conference at Parliament House in Canberra, Australia, last month. Abbott announced Sunday that his government would close a loophole to discourage families from refusing childhood vaccinations. Lukas Coch/EPA/Landov hide caption

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The H1N1 swine flu virus kills some people, while others don't get very sick at all. A genetic variation offers one clue. Centre For Infections/Health Pro/Science Photo Library/Getty Images hide caption

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Measles is highly contagious, and it produces fever and rash in susceptible people who become infected. Hazel Appleton/Health Protection Agency Centre/Science Source hide caption

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Students use tablets in a classroom in Mae Chan, a remote town in Thailand's northern province. Christophe Archambault/AFP/Getty Images hide caption

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Sara Martín reads bedtime stories with her children. When the kids were younger, she says, staying up to date on their frequent immunizations was tough, because of cost and transportation issues. Lauren M. Whaley/CHCF Center for Health Reporting hide caption

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Measles vaccine isn't a part of most workplaces. Eric Risberg/AP hide caption

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