Overview
Seasonal influenza is an acute viral infection caused by an influenza virus.
There are 3 types of seasonal 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. That is why only influenza A and B viruses are included in seasonal influenza vaccines.