Influenza viruses are differentiated on the basis of ribonucleoproteins into three types called A, B, and C. Type B and C are found mostly in humans, whereas type A is common in humans and many animals such as swine, horses and many birds. Influenza A viruses are classified into subtypes according to serological differences in their surface proteins hemagglutinin (H) and neuraminidase (N). Today 15 different H and 9 different N subtypes have been identified.
These surface glucoproteins permit the virus to attach to and infect susceptible hosts. The role of H is to initiate infection by docking onto the sialic acid surface of respiratory epithelial cells. The host cells enclose the viruses causing the viral and host cell membranes to fuse. This is followed by significant host immunologic responses.