Epidemiology of human influenza A and B viruses. Three different influenza A virus hemagglutinin subtypes (H1, H2, and H3) and two neuraminidase subtypes (N1 and N2) have been identified in humans over the last century. Although no live 1918 virus (solid square) is available, viral RNAs have been sequenced following RT-PCR from formalin-fixed or frozen tissue samples of 1918 victims (35). Rescue of virus containing reconstructed 1918 genes allows study of virulence characteristics of this long-gone strain (36).