Influenza is a contagious respiratory illness. It is characterized by a sudden onset of fever, headache, myalgia, malaise, prostration, non-productive cough, sore throat, and rhinitis. It has usually a short incubation period, from 1 to 3 days.
Most people recover within one to two weeks without any medical treatment. However, severe illness and death during annual influenza epidemics occur primarily among the very young, the elderly and those with other serious medical conditions.