Avian (or bird) influenza is a viral infection that can affect all species of birds. Some mammals can also be infected with avian influenza, including pigs and humans. A strain of avian influenza called H5N1 has caused illness and death in people in some areas of the world.

The first documented infection of humans with avian influenza virus occurred in Hong Kong in 1997, when the H5N1 strain caused severe respiratory disease. In February 2003, the H5N1 avian influenza resurfaced and caused an outbreak of disease.