How many flu strains are there?
Specialists say that there are 3 types of influenza viruses: A, B, and C. Believe it or not, combined, they are responsible for producing 306 human influenza viruses. Not too many people know that influenza A viruses are further classified by subtype on the basis of the two main surface glycoproteins hemagglutinin (HA) and neuraminidase (NA). The main idea is that influenza A subtypes and B viruses are further classified by strains. There are 16 known HA subtypes and 9 known NA subtypes.