The virus particle (virion) is 80–120 nanometers in diameter<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>.  The virion shape can be spherical, elliptical, or even filamentous with a length of tens of micrometers<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, Noda T., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>. The virion is made up of a viral envelope containing two main types of proteins, wrapped around a central core.  The two large proteins found on the outside of viral particles are hemagglutinin (HA) and neuraminidase (NA). HA is a protein that mediates binding of the virion to target cells and entry of the viral genome into the target cell, and therefore plays an important role in infecting healthy cells.  NA is involved in releasing the progeny viruses once a cell has been infected and has started producing the virus itself<ref name="Arti13Arti14">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>.