Flus are named for the properties of two key proteins that appear on their surfaces, hemagglutinin (H) and neuraminidase (N). The two numbers used to describe a strain of flu correspond to which variant of these two proteins a virus harbors. There are eighteen different subtypes of hemagglutinin and eleven different subtypes of neuraminidase. So, H5N1 is named for the fact that it combines the fifth subtype of hemagglutinin with the first subtype of neuraminidase.
