Influenza A virus subtype H5N1 (A H5N1 virus can also infect mammals (including humans) that have been exposed to infected birds; in these cases, symptoms are frequently severe or fatal.
A H5N1 influenza vary according to both the strain of virus underlying the infection and on the species of bird or mammal affected. Classification as either Low Pathogenic Avian Influenza (LPAI) or High Pathogenic Avian Influenza (HPAI) is based on the severity of symptoms in domestic chickens and does not predict the severity of symptoms in other species.