Influenza, commonly known as "the flu," is a highly contagious viral infection of the respiratory tract. It affects both sexes and all age groups, but its highest incidence is in children. Outbreaks tend to occur in the winter and early spring when as many as 40% of children can become infected.

There are three types of influenza viruses. Type A usually is responsible for the large influenza epidemics. Type A is constantly changing, with new strains appearing regularly. This results in a new epidemic every few years. Types B and C are not as widespread. Type B can cause smaller, more localized outbreaks. Type C is less common and usually causes only mild illness.

Influenza is caused by the influenza virus, which infects the respiratory tract (nose, throat, lungs). The flu usually spreads from person to person when an infected person coughs, sneezes, or talks and the virus is sent into the air. Unlike many other viral respiratory infections, such as the common cold, the flu causes severe illness and life-threatening complications in many people.

The flu differs in several ways from the common cold, a respiratory infection also caused by viruses. For example, people with colds rarely get fevers or headaches or suffer from the extreme exhaustion that flu viruses cause.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that 20 to 26 percent of Americans come down with the flu during each flu season, which typically lasts from November to March. Children are two to three times more likely than adults to get sick with the flu, and children frequently spread the virus to others. Although most people recover from the illness, CDC estimates that in the United States more than 100,000 people are hospitalized and about 50,000 to 70,000 people die from the flu and its complications every year.

Flu outbreaks usually begin suddenly and occur mainly in the late fall and winter. The disease spreads through communities creating an epidemic. During the epidemic, the number of cases peaks in about 3 weeks and subsides after another 3 or 4 weeks. Half of the population of a community may be affected. Because schools are an excellent place for flu viruses to attack and spread, families with school-age children have more infections than other families, with an average of one-third of the family members infected each year.

Besides the rapid start of the outbreaks and the large numbers of people affected, the flu is an important disease because it can cause serious complications. Most people who get the flu get better within a week (although they may have a lingering cough and tire easily for a while longer). For elderly people, newborn babies, and people with certain chronic illnesses, however, the flu and its complications can be life-threatening.