What does the 1 represent in H1N1?

Answer 1

It's the initials of two sugar proteins (their scientific names are hemagglutinin and neuramidinase) that sit on the surface of the virus and do its dirty work. There are 16 types of the H protein, numbered H1 through H16, and 9 types of the N protein, numbered N1 through N9. That makes 144 possible combinations of the virus, a constantly changing challenge for prevention or treatment. A new combination, H2N2, cause a brief Swine Flu epidemic in 1957. An H3N2 strain was the source of another epidemic in 1968. The bird flu virus that began in Southeast Asia a decade ago and has spread throughout the Old World is an H5N1 combination.

Answer 2

The entire name is simply a classification according to the subtype of the virus present. The 1 has no meaning on its own.