Influenza Virus - Trivial Pursuit level details...FYI

On a dark-blue background, this illustration provides a 3D graphical representation of a generic influenza virion’s ultrastructure, and is not specific to a seasonal, avian or 2009 H1N1 virus. A portion of the virion’s outer protein coat has been cut away, which reveals the virus’ contents. See PHIL 11877 for an uncut view of the virion's exterior, and PHIL 11879, which includes a key to the virion’s protein constituents.

There are three types of influenza viruses: A, B and C. Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States. The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H), and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.
 
Influenza B viruses are not divided into subtypes, however, influenza B viruses also can be further broken down into different strains.

CDC/Dough Jordan, M.A.; Photo: Dan Higgins; ID#: 11878 - This image is in the public domain and thus free of any copyright restrictions. www.cdc.gov