Flu

Influenza, or flu, is an acute respiratory infection caused by a variety of influenza viruses. The most familiar aspect of flu is the way it can "knock you off your feet" as it sweeps through entire communities, usually during the winter. Flu differs in several ways from the common cold, a respiratory infection also caused by viruses.

Significance

Outbreaks of flu usually begin abruptly. As the disease spreads through communities, the number of cases peaks in about 3 weeks and subsides after another 3 or 4 weeks. Twenty to fifty percent of a population may be affected, with the highest incidence in 5- to 14-year-olds. Schools are an excellent place for transmission of flu viruses, so that families with school-age children have a higher rate of infection than other families, with an average of one-third of the family members infected each year.

Besides the rapid onset of the outbreaks and the large numbers of people affected, flu is important because of the seriousness of the complications that can develop. Most people who contract the disease recover within a week (although they may tire easily for awhile). However, for elderly people, newborn babies, and people with certain chronic illnesses, flu and its complications can be life-threatening.

Transmission

Viruses that cause flu spread primarily from person to person, especially by coughing and sneezing (via airborne droplets of respiratory fluids). Flu viruses can enter the body through the mucous membranes of the eyes, nose, or mouth. After a person has been infected with the virus, symptoms usually appear within 2 to 4 days. The infection is considered contagious for another 3 to 4 days after symptoms appear.

The greatest risk of infection is in highly populated areas, where people live in crowded conditions, and in schools. Isolating people with flu symptoms is not an effective means of disease control because flu can be spread by someone whose symptoms are not yet apparent.

Symptoms

Flu is usually signaled by headache, chills, and dry cough, which are followed rapidly by body aches and fever. Typically, the fever starts declining on the second or third day of the illness. It is then that the upper respiratory symptoms become noticeable -- nasal congestion and sore throat. Flu almost never causes gastrointestinal symptoms; the illness that people often call "stomach flu" is not influenza.

Usually, doctors diagnose flu on the basis of whether flu is epidemic in the community and whether the patient's complaints fit the current pattern of symptoms. Doctors rarely use laboratory testing to identify the virus. Health officials monitor certain U.S. health clinics and do tests to determine which type of flu virus is responsible for the epidemic.