There are four types of influenza: A, B, C, and D, with A and B a focus among virologists and epidemiologists every year. Influenza A is divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). For example, H1N1 and H5N1 are both commonly circulating subtypes, with the former mostly infecting humans and the latter mostly infecting birds. Subtypes are further divided into variants, which can differ substantially in their transmissibility and risks.
