A prostitute in Johannesburg waits for a client on a street corner. An estimated two-thirds of sex workers in South Africa are HIV positive. Yoav Lemmer/AFP/Getty Images hide caption

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A nurse takes a blood sample from Nkosi Minenhle, 15, in a mobile clinic set up to test students for HIV at Madwaleni High School in KwaZulu Natal, South Africa. Stephane de Sakutin/AFP/Getty Images hide caption

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So cute, but not cuddly. The Egyptian tomb bat, Taphozous perforatus, is a likely carrier of the Middle East respiratory syndrome virus, or MERS. Courtesy of Jonathan H. Epstein/EcoHealth Alliance hide caption

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Somali women and children wait to get medicine in July 2008, from a clinic run by Doctors Without Borders about 20 miles south of Mogadishu. AFP/AFP/Getty Images hide caption

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A dromedary camel waits for a tourist to hop on its back in Petra, Jordan. The country has recorded two cases of the Middle East respiratory syndrome. Chris Jackson/Getty Images hide caption

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A red blood cell infected with malaria parasites (blue) sits next to normal cells (red). NIAID/Flickr.com hide caption

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Some scientists think new types of bird flus should arise only in chickens, not in labs. Here a worker collects poultry on a farm in Kathmandu, Nepal, where the H5N1 virus was infecting animals in October 2011. Prakas Mathema/AFP/Getty Images hide caption

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