Red Cross volunteers prepare to bury the body of an Ebola victim in Pendembu, Sierra Leone, early this month. Tommy Trenchard for NPR hide caption

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Usman (right), 7 months, and Abdullah (left), 18 months, are held by their mothers while they wait to receive the polio vaccine at the Jalozai refugee camp near Peshawar, Pakistan. Daniel Berehulak/Getty Images hide caption

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During nationwide polio campaigns, hundreds of thousands of health workers go door to door, giving children two drops of the polio vaccine. Anadolu Agency/Getty Images hide caption

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Medical workers treat Ebola patients at the Eternal Love Winning Africa hospital in Monrovia, Liberia. Three workers at the hospital, including Dr. Kent Brantly (left), have tested positive for Ebola. Courtesy of Samaritan's Purse hide caption

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Dr. Kent Brantly (right) of Samaritan's Purse gives orders to treat Ebola patients through the doorway of the isolation ward in Monrovia, Liberia. Courtesy of Samaritan's Purse hide caption

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Lucinda Schreiber for NPR

The CDC's director, Tom Frieden, testified before a congressional subcommittee Wednesday regarding a recent anthrax incident and lab safety improvements he is instituting. AFP/Getty Images hide caption

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Dogs throughout Latin America carry the Chagas parasite — and boost the risk of people catching it. And it's not just shelter dogs, like these in Mexico, who are at risk. Even family dogs get the deadly disease. Jose Luis Gonzalez /Reuters/Landov hide caption

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Suspicious travel companions: Bacteria can survive for days on surfaces inside a plane. But that doesn't mean you have to take these critters home with you. Benjamin Arthur for NPR hide caption

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Human immunodeficiency virus Type 1 inserts its genetic material into the DNA of human cells, turning them into little HIV factories. Eye of Science/Science Source hide caption

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