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.
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.
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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.
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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.
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.
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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.
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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.
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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.
A government official in Bali, Indonesia, holds a chicken before administering an injection to cull it as a precautionary measure in April to prevent the spread of bird flu.