Influenza, or flu, can be caused by different viruses of influenza: A, B, or C. Among those included in type A are viruses called bird, or avian, influenza viruses; A (H5N1) virus; or other nonhuman influenza subtypes (e.g., H7, H9).

Human infections with highly pathogenic avian influenza A (H5N1) virus occur through bird-to-human transmission; possibly environment-to-human transmission; and, very rarely, limited, nonsustained human-to-human transmission. However, in the first decade of the 2000s, outbreaks of human avian influenza in southeast Asia have been sustainably reported. Direct contact with infected poultry or with surfaces and objects contaminated by their droppings is the main route of transmission to humans. Exposure risk is considered highest during slaughter, defeathering, butchering, and preparation of poultry for cooking. There is no evidence that ...