3D graphical representation of a generic influenza virion's ultrastructure, and is not specific to a seasonal, avian or 2009 H1N1 virus. 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.