Influenzavirus A is a genus of the Orthomyxoviridae family of viruses.

Influenzavirus A includes only one species: Influenza A virus which causes influenza in birds and some mammals. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon.

Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans.Occasionally viruses are transmitted from wild aquatic birds to domestic poultry and this may cause an outbreak or give rise to human influenza pandemics.

Influenza A viruses are negative sense, single-stranded, segmented RNA viruses. There are several subtypes, labeled according to an H number (for the type of hemagglutinin) and an N number (for the type of neuraminidase).

There are 16 different H antigens (H1 to H16) and nine different N antigens (N1 to N9). The newest H type (H16) was isolated from black-headed gulls caught in Sweden and the Netherlands in 1999 and reported in the literature in 2005.

Each virus subtype has mutated into a variety of strains with differing pathogenic profiles; some pathogenic to one species but not others, some pathogenic to multiple species.