Biohazard suits used to handle dangerous microbes hang in a laboratory at the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Md. Patrick Semansky/AP hide caption

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An outbreak of bird flu in India in 2008 prompted authorities to temporarily ban the sale of poultry. Diptendu Dutta/AFP/Getty Images hide caption

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Particles of H5N1 virus — a particularly dangerous type of bird flu that can infect people — attack lung cells. Chris Bjornberg/Science Source hide caption

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Street vendors sell chickens at a market in Phnom Penh, Cambodia, in early 2013. Last year Cambodia reported more cases of H5N1 bird flu than any other country. Mak Remissa/EPA /LANDOV hide caption

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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

itoggle caption James Gathany/CDC/Douglas E. Jordan

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

itoggle caption Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases

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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