Influenza virus is an enveloped, single stranded RNA virus with a segmented genome. Virions are spherical to pleomorphic or filamentous, 80-120 nm diameter and 200-300 nm long. Surface features are distinctive spikes primarily of the hemagglutinin interposed irregularly with clusters of the neuraminidase protein. Strains of influenza A are by convention described by geographic origin, strain number, year of isolation and hemagglutinin (H) and neuraminidase (N) sub-types. Alternative Names: Influenza Type A virus protein, Flu A protein