Influenza A virus causes influenza in birds and some mammals. Influenza virus A is a genus of Orthomyxoviridae family of viruses. 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. Influenza A viruses are negative-sense, single-stranded, segmented RNA viruses.