Influenza, commonly called "flu", is an acute respiratory illness that affects the upper and/or lower parts of the respiratory tract and is caused by an influenza virus, usually of type A or B. Patients become ill between 18 and 72 hours after being infected. The most common symptoms of uncomplicated influenza are an abrupt onset of fever, shivering, headache, muscle ache and a dry cough. Cough is often severe and protracted, but other manifestations are usually self-limited with recovery in 2-7 days.