What's in a Flu Name "Where do terms like H1N1 and H7N9 come from?"

There are 198 possible subtypes of influenza A, among them H1N1 (pictured).

Q: Where do different flu strain names, like H1N1 and H7N9, come from?
— Claudia Mulder, Perth Amboy, NJ

A: First, let’s take a step back. Influenza viruses are divided into three categories: A, B and C, depending on their antigen protein type. (Antigens are any foreign substance that can stimulate an immune response, typically spurring antibody production.) Whereas type C can cause mild respiratory illness and B seasonal epidemics, type A can lead to serious worldwide pandemics and is the type you’re most likely to hear about in the news.

The World Health Organization, which adopted the classification system in 1979, further divided influenza A based on its surface glycoproteins, hemagglutinin (H) and neuraminidase (N). Hemagglutinin, which plays a role in how the virus binds to host cells, has 18 known types; neuraminidase, which helps the virus detach from the cell so it can spread through the body, has 11. The resulting 198 possible subtypes, from H1N1 to H18N11, could all be called “bird flu” because wild birds are influenza A’s natural reservoirs. A handful, however, can infect humans.