Influenza A is involved in most strains of the flu. It is an enveloped RNA virus. It has a protein membrane containing the glycoproteins hemagglutinin (HA) and neuraminidase (NA) which are used by the virus to enter a host cell and to release itself and its copies from the host cell. Each strain of the influenza virus has a different pattern of glycoproteins; the glycoproteins themselves have variability as well.