Highly pathogenic Influenza A (H5N1) virus also called HPAI H5N1 virus is an influenza A virus that occurs mainly in birds, is highly contagious among birds, and can be deadly to them, especially domestic poultry.vian influenza in birds. Avian influenza is an infection caused by avian (bird) influenza (flu) A viruses. These influenza A viruses occur naturally among birds. Wild birds worldwide get flu A infections in their intestines, but usually do not get sick from flu infections.