H5N1 influenza viruses cross the species barrier through close contact with infected birds, in particular domestic poultry. Contact with domestic poultry (chickens, ducks, etc.) has resulted in the inter-species transmission of H5N1 to non-avian hosts. Infrequently documented is the transmission of H5N1 from wild migratory birds to domestic and/or captive mammals. Though not documented, swine to human transmission of influenza subtypes H1N1, H2N2, and H3N2 does occur in nature.