Specific Avian Flu Viruses
Avian influenza A viruses do not normally infect people, but sporadic infections in people have occurred with some avian influenza A viruses. Illnesses in humans from avian influenza A virus infections have ranged in severity from asymptomatic to mild to severe and fatal disease. Human infections with avian influenza A viruses have most often occurred after contact with infected birds or their secretions or excretions. Three subtypes of avian influenza A viruses are known to infect people (H5, H7 and H9 viruses). Among these, Asian lineage H5N1 and H7N9 have caused the majority of infections in people.