Flu (influenza) viruses are divided into four broad categories: influenza A, B, C or D. Influenza A is the most common type. H1N1 flu is a subtype of influenza A.

Subtypes of influenza A are categorized based on two proteins on the surface of the virus, hemagglutinin (H) and neuraminidase (N). There are many H and N subtypes, and each one is numbered. All H and N flu subtypes are influenza A viruses.

Each influenza subtype has many different strains of influenza virus. Not all strains infect people. Subtypes of influenza A viruses currently found in people are strains of H1N1 and H3N2. Each year's flu vaccine includes varieties of both of these strains and influenza B.