Flu, also called influenza, is a viral infection of the nose, throat and lungs, which are part of the respiratory system. The viruses that cause influenza are divided into four categories named with letters. Influenza A is the most common type. The others are influenza B, C and D.

Influenza A viruses are further divided into subtypes. These subtypes are grouped based on two proteins on the surface of the viruses. One protein is called hemagglutinin (H) and the other is neuraminidase (N). There are many H and N subtypes and each one is numbered. H1N1 flu is a subtype of influenza A.

Each influenza subtype includes 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 these strains and influenza B.