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Influenza (flu) is important respiratory pathogens in mammals and can also produce highly fatal systemic disease in domestic poultry. Influenza A is found in a wide variety of bird and mammal species and is further divided into subtypes based on differences in the membrane proteins hemagglutinin (HA) and neuraminidase (NA). 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. Although avian influenza A viruses usually do not infect humans, rare cases of human infection with avian influenza A viruses have been reported. Most human infections with avian influenza A viruses have occurred following direct or close contact with infected poultry. Illness in humans has ranged from mild to severe.