The dynamics of human influenza strains appear to be driven by the need to evolve antigenic novelty (antigenic drift) to counteract herd immunity (see for example Garten et al., 2009; Boni, 2008; Russell et al., 2008; Smith et al., 2004), and the lineages of the internal gene segments broadly correspond to the lineages of the HA and NA genes, with very little reassortment between circulating subtypes. Here, XK is linked to influenza.