A boy in Lima, Peru, receives a hepatitis B vaccine during an immunization drive in 2008. The United Nations is considering a ban on the preservative thimerosal, which is often used in hepatitis B and other vaccines in developing countries. Martin Mejia/AP hide caption

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Does a little sugar water before shots really help ease a babies' pain? If only they could tell us. Dmitry Naumov/iStockphoto.com hide caption

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At a Los Angeles media briefing in 2010, Mariah Bianchi describes how her own case of whooping cough caused the death of her newborn son. Reed Saxon/AP hide caption

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A mother dresses her baby after doctors examined him during the malaria vaccine trial at the Walter Reed Project Research Center in Kombewa in Western Kenya in October 2009. Karel Prinsloo/AP hide caption

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Two boys study together at a Chicago yeshiva in 2009. Public health officials say this type of close physical contact caused a mumps outbreak to spread throughout several orthodox Jewish communities in and around New York City. M. Spencer Green/AP hide caption

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Health workers transport the polio vaccine by donkey in southeastern Pakistan. Asad Zaidi/Courtesy of UNICEF Pakistan. hide caption

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Nurse Fatima Guillen (left) gives 4-year-old Kimberly Magdeleno a whooping cough booster shot at a health clinic in Tacoma, Wash., in May. Ted S. Warren/AP hide caption

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A health worker in the Domincan Republic sprays insecticide between houses to stop dengue fever outbreaks this month. Erika Santelices/AFP/Getty Images hide caption

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