Two of the most important influenza surface proteins are hemagglutinin and neuraminidase, represented by the letters H and N, respectively, together with a number next to each letter. There are eighteen separate H proteins and eleven separate N proteins among influenza A viruses. Each virus has an H with a given number and an N with a given number. The pandemic of 1918 was caused by an influenza virus with the designation of H1N1. Influenza viruses did not start off evolutionarily as human viruses, and only a small proportion of these have infected humans. The remainder are animal influenzas,
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