Avian Influenza (Avian flu or Bird flu) refers to any influenza caused by viruses adapted to birds. It refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific type of host.

Avian flu can merge with other types of flu to create a new strain, as happened with the 2009 swine flu outbreak of H1N1. These new strains may be able to be transmitted to humans.

The highly pathogenic H5N1 virus is an emerging avian influenza virus that has been causing global concern as a potential pandemic threat. It is often referred to simply as "bird flu" or "avian influenza" even though it is only one subtype of avian influenza causing virus. Although millions of birds have become infected with the virus since its discovery, 248 humans have died from the H5N1 in twelve countries according to WHO data as of January 2009 [1].