Avian influenza a H5N1 virus

Although influenza A viruses of avian origin have long been responsible for influenza pandemics, including the " Spanish flu" pandemic of 1918, human infections caused by avian subtypes of influenza A virus, most notably H5N1, have emerged since the 1990s (H5N1 in 1997; H9N2 in 1999; and H7N7 in 2003). The wide geographic distribution of influenza A H5N1 in avian species, and the number and severity of human infections are unprecedented. Together with the ongoing genetic evolution of this virus, these features make influenza A H5N1 a likely candidate for a future influenza pandemic. This article discusses the epidemiology, pathogenesis, and diagnosis of human infections caused by influenza A H5N1 virus.