This also appears to be one factor determining the severity of infection by novel viral strains, although other properties of the HA of different strains and subtypes determine pathogenicity (Qi et al., 2014), and other viral genes play a role in pathogenicity as well (e.g., the NA and PB1 genes of the 1918 virus) (Kobasa et al., 2007). Here, XK is linked to infection.