The Influenza A virus, or Influenzavirus A, is a pathogen that causes the infectious influenza disease in birds and mammals. The virus is encoded by a single stranded RNA genome, typically spherical, and enveloped. Its hemagglutinin (HA) envelope glycoprotein mediates viral binding and entry into host cells. Another surface protein, neuraminidase (N), allows the release of the virion from the host. Subtypes of HA and N are used in classifying strains of influenza A. Notable strains that have spread to humans and have caused pandemics include H1N1, H2N2, H3N2, and H5N1.