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

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A well-wisher uses his phone to take a picture of a banner of photos of Nelson Mandela outside the Mediclinic Heart Hospital in Pretoria, where the former South African president is being treated. Ben Curtis/AP hide caption

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A Yemeni child receives a polio vaccine in the capital city of Sanaa. The Yemen government launched an immunization campaign last month in response to the polio outbreak in neighboring Somalia. Mohammed Huwais/AFP/Getty Images hide caption

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A woman waits to get AIDS drugs on April 8 at a clinic in Ga-Rankuwa, South Africa, about 55 miles north of Johannesburg. New WHO guidelines say patients should start HIV treatment much earlier, before they become extremely sick. Stephane de Sakutin/AFP/Getty Images hide caption

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A boy collects water at a new spigot in Shululu, Tajikistan. Before the government built a new water system, villagers were allocated half-hour time slots to collect water from a trickling tap. Jason Beaubien/NPR hide caption

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Rukshona Saidova, 12, lives with both HIV and tuberculosis. She can't walk right now because the diseases have atrophied muscles in her legs. Jason Beaubien/NPR hide caption

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Nurse Tina Martin checks on Orion Qurbonaliev, 4, who has tuberculosis. Orion's grandmother, Kholbibi Abdulloeva, also has TB. Jason Beaubien/NPR hide caption

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Health workers vaccinate a boy against polio at a May immunization drive in Mogadishu, Somalia. Farah Abdi Warsameh/AP hide caption

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Boys at the L'Ecole Les Freres Clement elementary school in Jacmel, Haiti, line up to take deworming pills that protect against elephantiasis. Maggie Steber for The Washington Post/Getty Images hide caption

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Students paste red ribbons on a window to mark World AIDS Day in Nanjing, China, in 2006. Although many infectious diseases have declined in the country, the number of new HIV cases nearly quadrupled between 2007 and 2011. AP hide caption

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