Three prominent subtypes of avian influenza A viruses are known to infect both birds and people.
Influenza A H5
There are nine known subtypes of H5 viruses (H5N1, H5N2, H5N3, H5N4, H5N5, H5N6, H5N7, H5N8, and H5N9). Most H5 viruses identified worldwide in wild birds and poultry are low pathogenic viruses, but occasionally highly pathogenic viruses have been detected. Sporadic H5 virus infection of humans, such as with Asian-origin highly pathogenic avian influenza A (H5N1) viruses currently circulating among poultry in Asia and the Middle East have been reported in 16 countries, often resulting in severe pneumonia with approximately 60% mortality worldwide.
Influenza A H7
There are nine known subtypes of H7 viruses (H7N1, H7N2, H7N3, H7N4, H7N5, H7N6, H7N7, H7N8, and H7N9). Most H7 viruses identified worldwide in wild birds and poultry are LPAI viruses. H7 virus infection in humans is uncommon, but has been documented in persons who have direct contact with infected birds, especially during outbreaks of H7 virus among poultry. Illness in humans may include conjunctivitis and/or upper respiratory tract symptoms.
In humans, LPAI (H7N2, H7N3, H7N7) virus infections have caused mild to moderate illness.
HPAI (H7N3, H7N7) virus infections have caused mild to severe and fatal illness.
On April 1, 2013, the first known human cases of infection with avian influenza H7N9 viruses were reported in China. These were associated with severe respiratory illness and death.
Influenza A H9
There are nine known subtypes of H9 viruses (H9N1, H9N2, H9N3, H9N4, H9N5, H9N6, H9N7, H9N8, and H9N9); all H9 viruses identified worldwide in wild birds and poultry are LPAI viruses. H9N2 virus has been detected in bird populations in Asia, Europe, the Middle East and Africa. Rare, sporadic H9N2 virus infections of humans have been reported to cause generally mild upper respiratory tract illness.