H5N1 is a subtype of the influenza A virus that can cause severe respiratory illness in humans and birds. H5N1 viruses are sometimes called avian influenza viruses because they primarily affect birds. However, these viruses can also infect pigs, humans, and other mammals.
The first human case of H5N1 infection was reported in 1997 in Hong Kong. Since then, there have been sporadic human cases of H5N1 infection reported from several countries in Asia, Africa, Europe, and the Middle East. The majority of these cases have been associated with contact with infected poultry or contaminated environments (such as live bird markets). Human-to-human transmission of H5N1 virus has occurred infrequently and has usually been limited to close contacts (such as family members) of people with H5N1 infection. There is no evidence that this virus spreads easily from person to person; however, limited spread among close contacts cannot be ruled out.
The current outbreak of avian influenza A(H7N9) began in China in February 2013. As of July 25, 2017, a total of 1,265 laboratory-confirmed cases of human infection with avian influenza A(H7N9) virus have been reported to WHO from mainland China and Hong Kong Special Administrative Region (SAR), including at least 425 deaths . Most patients had exposure to poultry or poultry markets before becoming ill; however, limited human-to-human transmission has occurred among close contacts such as family members.