How to understand influenza strain designations?

Question

What do the strain designations for flu mean? For example avian flu is classified as `H5N1`, what do the letters `H`, `N` and numbers `5`, `1` mean? Is it more than a simple string-identifier?

Answer 1

H is for Hemagglutinin, a protein on the surface of the influenza virus that allows it to attach to respiratory cells. N stands for Neuraminidase, an enzyme that helps the virus burst and exit cells where it has used the cell's machinery to make thousands of copies of itself.

There are 18 recognized H proteins and 11 recognized N proteins, for a grand total of 198 possible combinations. However, right now, only H1, H3 and H5 are known human pathogens (H1N1, H3N2, and H5N1, to be precise). But only H1 and H3 circulate seasonally right now. Others, like H7 are more predominant in birds.

This naming is only for type A influenza viruses. Type B viruses have a different naming convention.