influenza a virus subtype h5n1 influenza a virus subtype h5n1 influenza a virus subtype h5n1 (a/h5n1) is a subtype of the influenza a virus which can cause illness in humans and many other species. a bird-adapted strain of h5n1, called hpai a(h5n1) for highly pathogenic avian influenza virus of type a of subtype h5n1, is the highly pathogenic causative agent of h5n1 flu, commonly known as avian influenza ("bird flu"). it is enzootic (maintained in the population) in many bird populations, especially in southeast asia. one strain of hpai a(h5n1) is spreading globally after first appearing in asia. it is epizootic (an epidemic in nonhumans) and panzootic (affecting animals of many species, especially over a wide area), killing tens of millions of birds and spurring the culling of hundreds of millions of others to stem its spread. many references to "bird flu" and h5n1 in the popular media refer to this strain. influenza a virus subtype h5n1 colorized transmission electron micrograph of avian influenza a h5n1 viruses (seen in gold) grown in mdck cells (seen in green). virus classification (unranked): virus realm: riboviria kingdom: orthornavirae phylum: negarnaviricota class: insthoviricetes order: articulavirales family: orthomyxoviridae genus: alphainfluenzavirus species: influenza a virus serotype: influenza a virus subtype h5n1 notable strains goose guangdong virus a/fujian (h5n1) h5n1 influenza a virus subtype h5n1 genetic structure infection human mortality global spread in 2004 in 2005 in 2006 in 2007 social impact pandemic vaccine influenza (flu) types avian a/h5n1 subtype canine equine swine a/h1n1 subtype vaccines 2009 pandemic pandemrix live attenuated seasonal flu vaccine brands treatment amantadine baloxavir marboxil laninamivir oseltamivir peramivir rimantadine umifenovir zanamivir pandemics 1889-1890 russian flu 1918 spanish flu 1957-1958 asian flu 1968 hong kong flu 1977 russian flu 2009 swine flu outbreaks 1976 swine flu 2006 h5n1 india 2007 australian equine 2007 bernard matthews h5n1 2008 west bengal 2015 united states h5n2 outbreak 2020–2022 h5n8 outbreak