Bird flu (also known as avian influenza, avian flu, influenzavirus A, type A flu, or genus A flu) is a type of influenza virus that is hosted by birds, but may infect several species of mammals. It was first identified in Italy in the early 1900s and is now known to exist worldwide.

H5N1 is a highly pathogenic strain of bird flu. The first known appearance of this type of influenza in humans was in Hong Kong during 1997. The infection of humans coincided with an epidemic of avian influenza, caused by the same strain, in Hong Kong's poultry population. The outbreak was stopped by the killing of the entire domestic poultry population within the territory. The name H5N1 refers to the subtypes of surface antigens present on the virus: hemagglutinin type 5 and neuraminidase type 1. As of November 1, 2005, 122 cases of infections in humans, resulting in 62 deaths, have been confirmed outside of China. Thirteen countries across Asia and Europe have been affected. Additionally, more than 120 million birds have died from infection or been killed to prevent further infections.

Normally, avian flu viruses are transported worldwide in the intestines of wild birds, and are non-lethal. However, this variant has mutated into the most lethal strain of avian influenza ever recorded. Such mutations are natural and have happened in the past, as in the influenza pandemic caused by the 1918 Spanish flu, a variant of H1N1. Infected birds pass on H5N1 through their saliva, nasal secretions, and feces. Other birds may pick up the virus through direct contact with these excretions or when they have contact with surfaces contaminated with this material. Because migratory birds are among the carriers of the H5N1 virus it may spread to all parts of the world. Past outbreaks of avian flu have often originated in crowded conditions in southeast and east Asia, where humans, pigs, and poultry live in close quarters. In these conditions a virus can mutate into a form that more easily infects humans.

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