Influenza A viruses are assigned to groups according to the type of hemagglutinin (or H) and the type of the enzyme neuraminidase (or N) found on their surface. At the start of an infection, hemagglutinin plays a role in the attachment of the flu virus to the host cell. There are 15 known types of influenza hemagglutinins (or H1-H15). Neuraminidase promotes the release of newly formed influenza virus particles from infected cells. There are nine known varieties of neuraminidase—N1-N9. Major epidemics almost always coincide with a change in the hemagglutinin type or the neuraminidase type, or both, of the predominant circulating strain of influenza A virus. The predominant type in the United States this winter is H3N2.