Pathophysiology of Avian Influenza Avian Influenza is a zoonotic disease. It is transmitted from infected animals to healthy animals and humans. There has been no evidence of direct human-to-human transmission of Avian Influenza. An important protein called Hemagglutinin type 5 plays important character in the transmission of Avian Influenza. It attaches efficiently to the respiratory tract of aves whereas its efficiency is negligible in human beings. As far as human-to-human transmission of Avian Influenza is concerned, the role of sialic acid alpha(2-3) galactose is evident. This virus prefers it but it is available in the terminal bronchi and alveoli of human respiratory tract. When an infected human coughs or sneezes the respiratory droplets carrying the virus binds to the upper respiratory tract causing onset of infection.