Influenza viruses are divided in Alpha- Beta- Gamma- and Deltainfluenzavirus genus and are very similar in their overall structure.
The Influenzavirus A can be subdivided into different subtypes with their names based on the different structure of their important surfaceantigens hemagglutinin (HA) and neuraminidase (NA). This results in a serological classification with the pattern A/HxNx (simplified).
Responsible for the Influenza disease also commonly knows as "the flu" are mostly the eponymous Influenza A viruses or Influenza B viruses.
Influenza spreads around the world in yearly outbreaks and results in about three to five million cases of illness and about 200.000-650.000 deaths worldwide.
The influenza virus (mostly Type A) is also responsible for pandemics and epidemics in modern history and possibly even throughout human history.
One of the arguably most famous and lethal outbreak was the 1918 "spanish flu" a Type A / H1N1 subtype.