Influenza is an acute respiratory infection that can be caused by a variety of flu viruses. The virus spreads from person to person via airborne droplets of respiratory fluids, especially after an infected person coughs or sneezes. Flu viruses generally enter the body through the mucous membranes lining the eyes, nose and mouth. The primary symptoms include headache, chills and a dry cough, followed rapidly by body aches, malaise and fever. Typically, the fever starts dropping on the second or third day of the illness as upper respiratory symptoms, nasal congestion and sore throat become more prominent.
Most people recover from the flu within a week.
However, for those at high risk, such as the elderly and people with certain chronic illnesses, flu and its complications can be life threatening. Pneumonia generally results from secondary bacterial infections in the lower respiratory tract. In addition, a neurologic disease known as Reye's syndrome can develop in a small number of children and adolescents recovering from the flu. Reye's syndrome is associated with the use of aspirin, a component of many medications intended to relieve the pain or fever of the flu. Outbreaks of flu usually begin abruptly.