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Avian Influenza viruses are named according to the nature of two groups of proteins found on their surface - hemagglutinin(H) and neuraminidase (N). Hemagglutinin proteins are responsible for binding the virus to the cell that is being infected. Neuraminidase proteins aid in getting the virus into a host cell. There are 16 hemagglutinin and 9 neuraminidase types of proteins that can make up an avian influenza virus, giving rise to 144 possible combinations. For example, an H5N1 virus designates an avian influenza virus subtype that has an H5 protein and an N1 protein. All combinations of proteins may be found in wild aquatic birds, while H1, H2 and H3 have been circulating on and off in the human population for at least a century.