Avian influenza (bird flu) is an infectious disease that affects birds, caused by the influenza A virus. Birds are the hosts for a variety of influenza A virus subtypes, particularly waterfowl and shore birds. All birds are susceptible to avian influenza A and the virus can rapidly cause significant mortality in flocks but some species of bird show more severe clinical signs than others.
The influenza A viruses found in birds are not well adapted to infect humans, and human infections with avian influenza viruses are rare. However, some subtypes, such as A(H5N1) or A(H7N9), have been associated with human disease, usually in people directly exposed to infected birds or animals.
