Influenza A virus causes
influenza in birds and some
mammals, and is the only
species of influenza virus A. Influenza virus A is a
genus of the
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. Occasionally, viruses are transmitted from wild aquatic birds to domestic poultry, and this may cause an outbreak or give rise to human influenza
pandemics.