The flu is a contagious infection of the nose, throat, and lungs caused by the influenza virus.

See also: Swine flu (H1N1)

In temperate climates, influenza A usually arrives between early winter and early spring. Influenza B can appear at any time of the year.

The most common way to catch the flu is by breathing in droplets from coughs or sneezes. Less often, it is spread when you touch a surface such as a faucet handle or phone that has the virus on it, and then touch your own mouth, nose, or eyes.

Symptoms appear 1 - 7 days later (usually within 2 - 3 days). Because the flu spreads through the air and is very contagious, it often strikes a community all at once, causing an epidemic illness. This creates a cluster of school and work absences. Many students become sick within 2 or 3 weeks of the flu's arrival in a school.

Tens of millions of people in the United States get the flu each year. Most get better within a week or two, but thousands become sick enough to be hospitalized. About 36,000 people die each year from complications of the flu.

Sometimes people confuse colds and flu, which share some of the same symptoms and typically occur at the same time of the year. However, the two diseases are very different. Most people get a cold several times each year, and the flu only once every few years.

People often use the term "stomach flu" to describe a viral illness where vomiting or diarrhea is the main symptom. This is incorrect, as the stomach symptoms are not caused by the flu virus. Flu infections are primarily respiratory infections.