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

itoggle caption Diptendu Dutta/AFP/Getty Images

A cockerel walks on a bridge in a residential area of Beijing. The Chinese are beginning to destroy thousands of birds in an effort to stamp out the presumed source of H7N9 infection. Wang Zhao/AFP/Getty Images hide caption

itoggle caption Wang Zhao/AFP/Getty Images

H5N1 avian flu viruses (seen in gold) grow inside canine kidney cells (seen in green). Cynthia Goldsmith/CDC hide caption

itoggle caption Cynthia Goldsmith/CDC

H5N1 avian flu viruses (seen in gold) grow inside canine kidney cells (seen in green). Cynthia Goldsmith/CDC hide caption

itoggle caption Cynthia Goldsmith/CDC

H5N1 avian flu viruses (seen in gold) grow inside canine kidney cells (seen in green). Cynthia Goldsmith/CDC hide caption

itoggle caption Cynthia Goldsmith/CDC