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Phylogeny of HA gene sequences of H1N1 and H1N2 viruses infecting humans. Human cases of H1 swine that infected humans between 1976 and 2007 (black square); avian species (H1); seasonal human H1N1 influenza (black triangle), representative strains of Novel A/H1N1 influenza and H1N2 viruses (asterisk).
