Influenza, also known as the flu, is a contagious respiratory infection caused by different influenza viruses, according to WebMD. Muscle aches, headache and fever are symptoms of influenza.
There are three different types of flu viruses: A, B and C. Type A frequently mutates and can cause large flu epidemics. It can also cause infections in animals. Type B is less severe than type A and only infects humans. On occasion, it can cause large epidemics but not to the extent of type A. Type C also only infects humans. Type C does not cause large epidemics and does not affect people as strongly as either type A or B, notes WebMD.