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

itoggle caption ChinaFotoPress/Getty Images

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

itoggle caption Lam Yik Fei/Getty Images

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

itoggle caption AP

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

itoggle caption AFP/Getty Images

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

itoggle caption Eugene Hoshiko/AP

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

itoggle caption Prakas Mathema/AFP/Getty Images

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

itoggle caption Prakash Mathema/AFP/Getty Images

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

itoggle caption Andy Manis/AP

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

itoggle caption Prakash Mathema/AFP/Getty Images

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

itoggle caption Prakash Mathema /AFP/Getty Images

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

itoggle caption Ed Oudenaarden/AFP/Getty Images

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

itoggle caption Alexandre Meneghini/AP

National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said a voluntary halt to bird flu research should stay in effect. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

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. Firdia Lisnawati/AP hide caption

itoggle caption Firdia Lisnawati/AP