Avian influenza, also known as bird flu, is a subtype of the Influenza-A virus. The Influenza-A virus is the virus which causes influenza in birds. A specific subtype of this Influenza-A virus, named H5N1, is the virus which causes bird flu. The H5N1 virus has been known to infect large flocks of poultry birds in Europe, Asia, and Africa. This virus is also known to infect wild birds in these areas as well.
Historically, influenza viruses are believed to cycle from birds to swine and from swine to people.