Wild birds are the natural hosts of influenza A viruses. In principle, all subtypes of influenza A viruses can be found in them. Typically, little or no disease is caused by infection of wild birds with these avian influenza viruses, which are therefore called low pathogenic avian influenza (LPAI) viruses. However, when LPAI viruses of the H5 or H7 subtypes are transmitted to domestic poultry, such as turkeys and chickens, they may acquire mutations in their HA, by which they change into so-called highly pathogenic avian influenza (HPAI) viruses. These HPAI viruses then cause very severe disease outbreaks in domestic poultry, of which up to one hundred percent may die.
Such viruses can also cause serious disease and death when transmitted to wild birds, although some avian species have recently been shown to be less susceptible.