Influenza is an acute respiratory illness caused by infection with an influenza virus.
A third type, influenza C, is rarely reported as a cause of human illness. Influenza is characterised by sudden onset of symptoms with patients often recalling the exact hour the fever commenced. The symptoms include a temperature of 38 C or more with a dry cough, headache, sore muscles and sore throat. The most frequent complication is pneumonia, most commonly secondary bacterial pneumonia. Primary influenza viral pneumonia is an uncommon complication but is associated with a high death rate.