H5N1 Avian Flu (H5N1 Bird Flu). H5N1 is a highly pathogenic avian (bird) flu virus that has caused serious outbreaks in domestic poultry in parts of Asia and the Middle East.Highly pathogenic refers to the virusâs ability to produce disease.5N1 Avian Flu (H5N1 Bird Flu). H5N1 is a highly pathogenic avian (bird) flu virus that has caused serious outbreaks in domestic poultry in parts of Asia and the Middle East.