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

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.

The flu usually begins abruptly, with a fever between 102 and 106 °F. (An adult typically has a lower fever than a child.) The fever usually lasts for a day or two, but can last 5 days.

Other common symptoms include:

• Body aches
• Chills
• Dizziness
• Flushed face
• Headache
• Lack of energy
• Nausea
• Vomiting

Somewhere between day 2 and day 4 of the illness, the "whole body" symptoms begin to subside, and respiratory symptoms begin to increase.

The most prominent of the respiratory symptoms is usually a dry, hacking cough. Most people also develop a sore throat and headache. Runny nose (nasal discharge) and sneezing are common.

These symptoms (except the cough) usually disappear within 4 - 7 days. Sometimes, the fever returns. The cough and tiredness usually last for weeks after the rest of the illness is over.

Other symptoms may include:

• Loss of appetite
• Muscle aches and stiffness
• Stuffy, congested nose
• Sweating
• Worsening of underlying illness, such as asthma or heart failure

The evaluation of an individual with flu symptoms should include a thorough physical exam and, in cases where pneumonia is suspected, a chest x-ray.

Additional blood work may be needed. They may include a complete blood count, blood cultures, and sputum cultures.

The most common method for diagnosing the flu is an antigen detection test, which is done by swabbing the nose and throat, then sending a sample to the laboratory for testing.

The results of these tests can be available rapidly, and can help decide if specific treatment is appropriate. However, when flu is widespread in the community the diagnosis can often be made by simply identifying symptoms without further testing.

If you have mild illness and are not at high risk, take these steps:

• Rest
• Take medicines that relieve symptoms and help you rest
• Drink plenty of liquids
• Avoid aspirin (especially teens and children)
• Avoid alcohol and tobacco
• Avoid antibiotics (unless necessary for another illness)

If the flu is diagnosed within 48 hours of when symptoms begin, especially if you are at high risk for complications, antiviral medications may help shorten the length of symptoms by about one day.

Treatment is usually not necessary for children, but if the illness is diagnosed early and the patient is at risk of developing a severe case, it can be started.

Treatment will only help if started early and only if the illness is actually influenza. It will not help treat a regular cold.

Anyone at any age can have serious complications from the flu, but those at highest risk include:

• People over age 50
• Children between 6 months and 2 years
• Women more than 3 months pregnant during the flu season
• Anyone living in a long-term care facility
• Anyone with chronic heart, lung, or kidney conditions, diabetes, or a weakened immune system

In most individuals who are otherwise healthy, the flu goes away within 7 to 10 days.

Possible complications, especially for those at high risk, include:

• Pneumonia
• Encephalitis (infection of the brain)
• Bronchitis
• Sinus infections
• Ear infections

Call your health care provider if someone in a high-risk category develops symptoms of the flu, or if your illness seems severe.

A yearly vaccine is recommended for children older than 6 months, adolescents, and adults.

The vaccine is available as a flu shot or a nasal spray-type flu vaccine.

For specific recommendations, see influenza vaccine.

Fiore AE, Shay DK, Broder K, et al. Prevention and control of influenza. Recommendations of the Advisory Committee on Immunization Practices. August 8, 2008. 57(RR07):1-60.

American Academy of Pediatrics. Prevention of Influenza: Recommendations for Influenza Immunization in Children, 2008-2009. Accessed November 9, 2008.

Jefferson T, Demichelli V, Rivetti D, Jones M, Di Pietrantoni C, Rivetti A. Antivirals for influenza in healthy adults: systematic review. Lancet. 2006;367:303-313.

Beigel JH, Dellinger RP. Influenza. Crit Care Med. 2008;36.