In 1997, 18 human infections with H5N1 influenza type A were identified in Hong Kong and six of the patients died. There were concomitant outbreaks of H5N1 infections in poultry. The gene segments of the human H5N1 viruses were derived from avian influenza A viruses and not from circulating human influenza A viruses. In 1999 two cases of human infections caused by avian H9N2 virus were also identified in Hong Kong. These events established that avian influenza viruses can infect humans without passage through an intermediate host and without acquiring gene segments from human influenza viruses. The likely origin of the H5N1 viruses has been deduced from molecular analysis of these and other viruses isolated from the region. The gene sequences of the H5N1 viruses were analysed in order to identify the molecular basis for the ability of these avian viruses to infect humans.
Copyright 2000 John Wiley & Sons, Ltd.