Avian influenza, more commonly called "bird flu," is a type of influenza or "flu" that occurs in all species of birds. The virus that causes avian influenza exists naturally in many wild birds, including wild waterfowl, without causing the condition in them (these birds are called carriers). The virus is usually associated with birds raised on poultry farms.
Most people may not immediately make the connection between avian influenza and humans. Avian influenza is rare in humans. When it does infect a human, the virus often causes serious illness or death.
Avian influenza has been getting increasing worldwide attention from medical authorities. A new subtype of the virus called H5N1, which first infected chickens and then humans in Hong Kong in 1997, can cause disease with a high rate of mortality (death) in humans.