 Two main types of influenza virus--Types A and B--are responsible for the seasonal flu epidemics that occur each year. The influenza A viruses can be broken down into subtypes based on two proteins on the surface of the virus: hemagglutinin H and neuraminidase N. The A subtypes usually found in humans are H1N1, H1N2, and H3N2. Other A subtypes are found primarily in animals. For example, the avian influenza virus refers to an influenza A virus that is found chiefly in birds. 