A nurse vaccinates a child against pneumonia at a healthcare center in Managua in January. Nicaragua received pneumococcal vaccines from the GAVI Alliance. ELMER MARTINEZ/AFP/Getty Images hide caption

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A man readies a cow for the International Highland Cattle Show in Glasgow, Scotland. Researchers say genetics and the amount of time animals and humans spend together can affect how viruses spread between species. Jeff J Mitchell/Getty Images hide caption

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Laurence Fishburne as Dr. Ellis Cheever and Kate Winslet as Dr. Erin Mears in the thriller Contagion. Winslet's character was modeled on CDC epidemiologist Dr. Anne Schuchat. Claudette Barius/Warner Bro. Pictures hide caption

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Researchers hope to keep the mosquito that transmits dengue, Aedes aegypti, from infecting humans using the Wolbachia bacterium. James Gathany/CDC Public Health Image Library hide caption

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Women hold mosquito nets after receiving them at a distribution point in Sesheke, Zambia. Researchers say malaria may have rebounded in some parts of Zambia and Senegal because of resistance to the insecticide-treated nets. ASSOCIATED PRESS hide caption

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Indian schoolchildren write English alphabets on slates at a primary school outside Hyderabad in June. India is on track to overtake China as the most populous nation in just 16 years. Noah Seelam/AFP/Getty Images hide caption

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An Afghan health worker administers the polio vaccine to a child during a vaccination campaign in Kabul, Afghanistan, in 2010. Majid Saeedi/Getty Images hide caption

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A study of Ugandans found antiretroviral drugs can extend the lifespans of people with HIV to nearly normal lengths. Adek Berry/AFP/Getty Images hide caption

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Mourners attend a memorial service for recent cholera victims in Haiti that took place Wednesday in Savanette, an isolated community in the mountains outside Mirebalais, Haiti. Cate Oswald/Partners in Health hide caption

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A man pushes a friend in a wheelchair following an event in Beijing to raise awareness of accessibility issues for the disabled. Ng Han Guan/ AP Photo hide caption

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Lettuce at an organic vegetable farm in Teltow, Germany. Organic farmers in Germany are reporting a surge in demand for lettuce and cucumbers in the wake of an outbreak of E. coli after health officials warned people not to eat cucumbers, lettuce and tomatoes, though organic produce may not necessarily be safer. Sean Gallup/Getty Images hide caption

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