XK and infection: There are 18 known HA subtypes (H1–H18)and 11 known NA subtypes (N1–N11)[13].Precursors to future pandemics could be viruses carryingthe HA subtypes H1, H2, H3, H5, H6, H7, H9, H10, and NA subtypes N1, N2, N3, N8that have been known to cause outbreaks or sporadic human infections.