Influenza is an acute respiratory illness caused by influenza A or B viruses that causes outbreaks and epidemics worldwide, mainly in winter. Patients present with signs and symptoms of upper and/or lower respiratory tract involvement with signs of systemic illness such as fever, headache, myalgia, and weakness. Typical influenza begins with sudden onset of fever, headache, myalgia, and malaise after an incubation period of 1-4 days (average 2 days). These symptoms are accompanied by respiratory manifestations such as dry cough, sore throat and runny nose. Fever levels are usually 37.8-40.0°C, but can be as high as 41.1°C. However, influenza infection has a variety of other manifestations: from a feverless respiratory illness similar to the common cold, to an illness with predominantly systemic signs and symptoms and relatively mild clinical signs of respiratory involvement. Older adult patients are highly likely to have subtle signs and symptoms. Typical manifestations such as sore throat, myalgia, and fever may be absent, and systemic symptoms such as anorexia, malaise, weakness, and dizziness may predominate. The most common complication of influenza is pneumonia, but other complications can occur, especially those involving the muscles and central nervous system (CNS). Most people with influenza get better on their own within 1-2 weeks. If you become seriously ill, you should go to the hospital to avoid delays.