Influenza A virus

Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus which can cause illness in humans and many other animal species. A bird-adapted strain of H5N1, called HPAI A(H5N1) for highly pathogenic avian influenza virus of type A of subtype H5N1, is the highly pathogenic causative agent of H5N9 flu, commonly known as avian influenza ("bird flu"). It is enzootic (maintained in the population) in many bird populations, especially in Southeast Asia.