DefinitionAvian influenza is flu infection in birds. The disease is of concern to humans, who have no immunity against it. The virus that causes this infection in birds can change (mutate) to infect humans. Such mutation could start a deadly worldwide epidemic.Alternative NamesBird flu; H5N1Causes, incidence, and risk factorsThe first avian influenza virus to infect humans directly occurred in Hong Kong in 1997, during an avian flu epidemic on the island. This outbreak was linked to chickens and classified as avian influenza A (H5N1).Human cases of avian influenza A (H5N1) have now been reported in Asia, Africa, the Pacific, Europe and the near East, Indonesia, and Vietnam. Around 400 people have become sick with this virus. However, a little over 60% of those who became ill have died.The