A vendor sells chickens at the Kowloon City Market in Hong Kong last month. As a precautionary measure against the deadly H7N9 virus, Hong Kong has temporarily stopped importing poultry from mainland farms. Lam Yik Fei/Getty Images 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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A man who just recovered from the H7N9 bird flu leaves a hospital in Bozhou, China, in April. Since early May, the number of new H7N9 cases has dramatically declined. AFP/Getty Images hide caption

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Men outside a hospital in Dammam, Saudi Arabia, wear surgical masks as a precaution against infection with a coronavirus. Stringer/Reuters /Landov hide caption

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A vendor weighs a live chicken at the Kowloon City Market in Hong Kong last April. After closing live poultry shops in many cities around China, the rate of new H7N9 infections sharply declined. Lam Yik Fei/Getty Images hide caption

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Scientists in the U.S. are growing the H7N9 virus in the laboratory to help with vaccine development. James Gathany/CDC/Douglas E. Jordan hide caption

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Influenza covers it's shell with two types of accessories: the H spike, blue, and the N spike, red. Here the flu particle is sliced open to show its genetic material. Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases hide caption

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A health worker collects pigeons from a trap at People's Square in Shanghai, China, earlier this month. So far, workers have tested more than 48,000 animals for the H7N9 flu virus. ChinaFotoPress/Getty Images hide caption

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A vendor weighs a live chicken at the Kowloon City Market in Hong Kong Friday. Health authorities there have stepped up the testing of live poultry from China to include a rapid test for the H7N9 bird virus. Lam Yik Fei/Getty Images hide caption

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People sit near pigeons at a park in Shanghai Sunday. A new strain of bird flu has spread from eastern China to other provinces, with 13 deaths reported. AP hide caption

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Workers prepare an H7N9 virus detection kit at the Center for Disease Control in Beijing on April 3. AFP/Getty Images hide caption

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A child wears a mask near a closed section of a poultry market in Shanghai, where health workers detected the new bird flu, H7N9. Eugene Hoshiko/AP hide caption

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Health officials around the world are on constant lookout for the deadly bird flu. Here a worker collects chickens on a farm in Kathamndu, Nepal, where the virus was suspected of infecting poultry last October. Prakas Mathema/AFP/Getty Images hide caption

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Health workers in Nepal culled chickens and destroyed eggs following an outbreak of bird flu in Kathmandu in October 2012. Prakash Mathema/AFP/Getty Images hide caption

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Researchers at the U.S. Geological Survey National Wildlife Health Center in Madison, Wis., use eggs to see if the Asian strain of the H5N1 bird flu virus has entered the U.S. in this photo from 2006. Andy Manis/AP hide caption

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A health official culls chickens on a poultry farm in a village on the outskirts of Katmandu, Nepal. Chickens suspected of being infected with H5N1 bird flu were found in the area in October. Prakash Mathema/AFP/Getty Images hide caption

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A prefectural officer carries a chicken on a poultry farm on Oct. 15 on the outskirts of Kathmandu, Nepal, where chickens suspected of being infected with bird flu were found. Prakash Mathema /AFP/Getty Images hide caption

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When a case of the potentially lethal H5N1 bird flu was found in British poultry in 2007, Dutch farmers were told to keep their poultry away from wild birds by closing off outdoor areas with wire mesh. Ed Oudenaarden/AFP/Getty Images hide caption

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A city worker sells eggs as people line up outside the truck in Mexico City on Aug. 24. The Mexican government is battling an egg shortage that has caused prices to spike in a country with the highest per-capita egg consumption on Earth. Alexandre Meneghini/AP hide caption

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