Avian influenza virus contains hemagglutinin 5 which attaches to the respiratory epithelium. Soon, it begins spreading among avian species. Surprisingly, its transmission and attachment to human respiratory epithelium is more difficult. Avian influenza virus requires acid alpha(2-3) galactose. It is present in the human respiratory tract at terminal bronchi and alveoli. However, human viruses require acid alpha(2-3) galactose. It is present in the human respiratory tract in e[ithelial cells on upper respiratory tract. This explains why there is very limited number of reported cases due to human-to-human interactions.