Microsoft founder Bill Gates, his wife Melinda French Gates and British Prime Minister David Cameron at the World Economic Forum on January 28, 2011, in Davos, Switzerland. Johannes Eisele/AFP/Getty Images hide caption

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A student at Carlin Springs Elementary School receives a flu shot last January in Arlington, Va. Mandel Ngan/AFP/Getty Images hide caption

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A study that measured antibodies and the disease history of more than  200 kids found that two doses of vaccine were 98 percent effective in  preventing chickenpox compared with 86 percent for a single dose. AP hide caption

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A 10-year-old Somali boy being treated for meningitis in May 2009 at a makeshift clinic in Mogadishu. ROBERTO SCHMIDT/AFP hide caption

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Nicole Andreacchio, second from the right, was seven months pregnant when she went to get vaccinated against swine flu in Maple Glen, Pa., in October 2009. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

A man helps his dengue fever-stricken daughter in Tegucigalpa, Honduras. In June, Honduran authorities declared a state of emergency due to the high rate of dengue cases reported around the country. Orlando Sierra/AFP hide caption

itoggle caption Orlando Sierra/AFP