Influenza or "flu" is caused by a virus. The flu virus has only 8 genes, and two of them, abbreviated H and N are responsible for infecting us.
There are 16 different H genes and 9 different N genes. The flu virus has the clever ability to mix and match different combinations to come up with new viruses.
Different flu strains are named after the various combinations of the H and N genes.
- Spanish flu (1917-1918) is called the H1N1 strain
- Hong Kong flu (1968-1969) was caused by the H3N2 strain
When we make antibodies to the proteins that the H and N genes make, the antibodies protect us from being infected with the same strain of virus a second time.