Influenza, also known as the ‘flu,’ is a contagious disease caused by infection with the influenza virus. It is a common disease, with annual attack rates of 10% to 30% worldwide each year, with most cases in the northern hemisphere occurring during the ‘flu season’ of December through March. Flu is characterized by fever, by respiratory symptoms, including rhinorrhea, cough, and sore throat, and sometimes by myalgia and headache. In children and infants, gastrointestinal symptoms such as vomiting and diarrhea occur in 50% of cases, but such symptoms are rare in adult cases. The course of influenza is usually self-limiting (i.e., resolves without medical intervention) and lasts [Page 534]3 to 5 days, but serious complications, including pneumonia, may develop that prolong the illness and may ...