Helen Down holds her 14-month-old daughter, Amelia, for an MMR shot in Swansea, Wales, April 2013. The vaccination was in response to a measles outbreak. Geoff Caddick/AFP/Getty Images hide caption

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Marqui Ducarme is aided by his wife after catching chikungunya at his home in Port-au-Prince, May 23. The virus swept through Haiti this spring, infecting more than 40,000 people. Marie Arago/Reuters/Landov hide caption

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Red blood cells infected with the Plasmodium falciparum parasite. Plasmodium is the parasite that triggers malaria in people. Gary D. Gaugler/Science Source hide caption

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A doctor gives a polio vaccine to a child at a health clinic in Baghdad last week. The CIA says it banned the use of vaccine programs as cover for spying last year — a practice health officials said had wide repercussions. Ahmad Al-Rubaye/AFP/Getty Images hide caption

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False-color transmission electron micrograph of a field of whooping cough bacteria, Bordetella pertussis. A. Barry Dowsett/Science Source hide caption

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There today, here tomorrow: A mother holds her child for a measles vaccination in Manila, Philippines, in January. Travelers are bringing measles from the Philippines to the United States. Noel Celis/AFP/Getty Images hide caption

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This one's virus-free: Matthew Followill, Nathan Followill and Caleb Followill of Kings of Leon performed in Los Angeles in December. Kevin Winter/Getty Images for Radio.com hide caption

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A school nurse prepares a vaccine against whooping cough before giving it to students at Mark Twain Middle School in Los Angeles. Kevork Djansezian/Getty Images hide caption

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Princeton University's Nassau Hall. DANIEL HULSHIZER/AP hide caption

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Public school student Julio Valenzuela, 11, grimaces as he gets a vaccination before the start of the school in Lynnwood, Calif., on Aug. 27. Vaccines are required for school attendance. ROBYN BECK/AFP/Getty Images hide caption

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Avez, 2, is held by his mother, as he receives the polio vaccine in the Khyber Tribal Region in northwest Pakistan. Daniel Berehulak/Getty Images hide caption

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A student gets vaccinated against pertussis at a Los Angeles middle school in 2012. The state required that students be immunized to halt an epidemic of whooping cough. Kevork Djansezian/Getty Images hide caption

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