Bacteroides are microbes that dominate the guts of people who eat more animal protein and fat. National Institute of Environmental Health Sciences hide caption

itoggle caption National Institute of Environmental Health Sciences

A silicone gel implant (left) and a saline gel implant. The FDA agreed to let implants return to the market in 2006, but stipulated that manufacturers track long-term side effects. Spencer Platt/Getty Images hide caption

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A lab technician prepares blood samples from volunteers for viral genotyping at a government-run health center in Bagamoyo, Tanzania, in 2009. Tanzania is currently hosting the final stages of a human trial of a pioneering vaccine against malaria. The vaccine is one of many medical innovations to emerge from Walter Reed over the decades. Tony Karumba/AFP/Getty Images hide caption

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Two women check their cellphones as they hawk their wares on a bridge over the Artibonite River, whose waters are believed to be the source of Haiti's 2010 cholera outbreak. Nicholas Kamm/AFP/Getty Images hide caption

itoggle caption Nicholas Kamm/AFP/Getty Images