Influenza A (H5N1) is a subtype of the Type A influenza virus. Wild birds are the natural hosts of the virus, hence the name avian influenza or bird flu. The virus was first isolated from birds (terns) in South Africa in 1961. The virus circulates among birds worldwide. It is very contagious among birds and can be deadly to birds, particularly domesticated birds like chickens The virus does not typically infect humans. In 1997, however, the first instance of direct bird-to-human transmission of H5N1 was documented during an outbreak of avian influenza among poultry in Hong Kong; the virus caused severe respiratory illness in 18 people, of who 6 died. But so far, H5N1 viruses have not been capable of efficient human-to-human transmission. Avian influenza viruses spread among susceptible birds when they have contact with contaminated excretions (saliva, nasal secretions, and feces). It is believed that most cases of H5N1 infection in humans have resulted from contact with infected poultry or contaminated surfaces. | 
