Influenza A viruses are negative-sense, single-stranded, segmented RNA viruses. They are the only species of the genus Alphainfluenzavirus of the virus family Orthomyxoviridae

Influenza A virus causes influenza in birds and some mammals. Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans. However, over the past few years, various subtypes of Influenza A viruses, such as H5N1, have been reported to infect humans.