What does H1N1 stand for?
Influenza A virus strains are categorized according to two proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N), according to Maggie Thompson, director of communications with the Kansas Department of Health and Environment. All influenza A viruses contain hemagglutinin and neuraminidase, but the structures of these proteins differ from strain to strain, because of rapid genetic mutation in the viral genome.
Influenza A virus strains are assigned an H number and an N number based on which forms of these two proteins the strain contains. There are 16 H and nine N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.