Why H1N1, H5N1,...?

Influenza A virus strains are categorized qccordind to two protein found on the surface of the virus: henaglutinnins (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins differs from strain to strain due to 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 9 N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans


The various types of influenza viruses in humans. Solid squares show the appearance of a new strai, causing recurring influenza pandemics. Broken lines indicate uncertain strain identifications