That last post got me curious about how the name “H5N1” came about. The Centers for Disease Control and Prevention explains:

“Influenza type A viruses are divided into subtypes and named on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, an ‘H7N2 virus’ designates an influenza A subtype that has an HA 7 protein and an NA 2 protein. Similarly an ‘H5N1’ virus has an HA 5 protein and an NA 1 protein. There are 16 known HA subtypes and 9 known NA subtypes. Many different combinations of HA and NA proteins are possible.”

Also, while we’re here: The Big Picture Book of Viruses.