The term “H5N1” indicates a virus featuring subtype 5 of the hemagglutinin protein combined with subtype 1 of the neuraminidase protein. Sixteen different kinds of hemagglutinin, plus nine kinds of neuraminidase, have been detected in the natural world. Hemagglutinin is the key that unlocks a cell membrane so that the virus can get in, and neuraminidase is the key for getting back out.
