XK and influenza: So far, 18 HA subtypes (H1–18) and 11 NA subtypes (N1–N11) were identified [4], in which several different subtypes caused severe pandemics in the last century, including the 1918 Spanish flu (H1N1), 1957 Asian flu (H2N2), 1968 Hong Kong flu (H3N2) [5], and 2009 swine flu (H1N1 pdm09) [6].