Influenza, or flu, is a respiratory infection caused by several flu viruses. Like the common cold, it infects the nose, throat, and lungs. 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. The best way to prevent the flu is by getting a flu vaccine every year.
The flu differs in several ways from the common cold (PDF). For example, people with colds rarely get fevers or headaches or suffer from the extreme exhaustion that flu viruses cause. The most familiar aspect of the flu is the way it can "knock you off your feet" as it sweeps through entire communities.
The Centers for Disease Control and Prevention (CDC) estimates that 5 to 20 percent of Americans come down with the flu during each flu season, which typically lasts from October 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.
For elderly people, newborn babies, pregnant women, and people with certain chronic illnesses, the flu and its complications can be life-threatening. Although most people recover from the illness, CDC estimates that between 3,000 and 49,000 Americans die from the flu and its complications every year.
Seasonal flu refers to the flu outbreaks that occur each year, 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 three weeks and subsides after another three to four weeks.
Pandemic flu refers to particularly contagious strains of flu that spread rapidly from person to person to create a worldwide epidemic (pandemic). In the past century, there were influenza pandemics in 1918, 1957, 1968, and 2009.
Last Updated November 16, 2012