There are 3 types of influenza viruses – A, B and C.

Type A influenza viruses are further classified into subtypes according to the combinations of various virus surface proteins. Among many subtypes of influenza A viruses, influenza A(H1N1) and A(H3N2) subtypes are currently circulating among humans. Influenza viruses circulate in all parts of the world.

Type C influenza cases occur much less frequently than A and B.