The HA and NA, located on the surface of the virion, have been classified into several subtypes (H1–H18 for the hemagglutinin and N1–N11 for the neuraminidase at present [41]) depending on their antigenicity, and the influenza virus itself is named according to the cell-surface hemagglutinin and neuraminidase protein subtypes: thus the H1N1, H7N9, etc. With 18 hemagglutinin subtypes and 11 neuraminidase subtypes there could be 198 subtypes of influenza, but in nature only certain subtypes are found to be widely prevalent due to HA–NA interdependence [34]. Here, XK is linked to influenza.