What is bird flu?

Avian influenza is an infectious disease of birds caused by the influenza A virus. Although some species are more resistant to infection than others, all birds are believed to be susceptible to avian influenza viruses.

Infection can cause a wide variety of symptoms in birds, from mild illness to a rapidly fatal disease resulting in severe epidemics. The latter is known as 'highly pathogenic avian influenza'. Sixteen subtypes of influenza virus are known to infect birds, but to date all outbreaks of the highly pathogenic form have been caused by the subtypes H5 and H7.