The historic influenza panda mix

There have been four flu pandemics so far, named for their country of origin:

• 1889 Russian flu, estimated one million deaths, possible H3 strain
• 1918 Spanish flu, estimated 20 to 100 million deaths, H1N1 strain
• 1957 Asian flu, estimated one million deaths, H2N2 strain
• 1968 Hong Kong flu, estimated one million deaths, H3N2 strain

BBC News: The mother of all flu pandemics.

Viral strains take their name from the various different types of two important proteins on their surface. Their initials are H for haemaglutanin, and N for neuraminidase.  The haemaglutanin molecule protrudes from the surface of the virus as spikes, and enables it to lock on to receptors on the surface of cells. Neuraminidase allows the virus to be released from infected cells and infect new ones.

"There are 16 Hs and 9 Ns, but as far as we know, only H1,2,3 and only N1 and N2 have ever made it into a human virus," says Dr Morens.

In theory, that means we only have six variants of flu to fight.  Dr Morens said "In the category of Influenza A, which is the category of virus that has caused all human epidemics and pandemics, every virus circulated since 1918 has been a descendent of this virus in one way or another."  Swine flu is a previously unknown H1N1 strain which I think ought to be named Mexican flu.  The avian flu scare of 2006-2008 was an H5N1 flu.