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

Alternative Names

Flu; Influenza A; Influenza B


The flu usually begins abruptly, with a fever between 102 to 106°F. (An adult typically has a lower fever than a child.) Other common symptoms include a flushed face, body aches, and lack of energy. Some people have dizziness or vomiting. The fever usually lasts for a day or two, but can last 5 days.

Somewhere between day 2 and day 4 of the illness, the "whole body" symptoms begin to subside, and respiratory symptoms begin to increase. The flu virus can settle anywhere in the respiratory tract, producing symptoms of a cold, croup, sore throat, bronchiolitis, ear infection, or pneumonia.

The most prominent of the respiratory symptoms is usually a dry, hacking cough. Most people also develop a sore throat and headache. Nasal discharge (runny nose) and sneezing are common. These symptoms (except the cough) usually disappear within 4-7 days. Sometimes, the fever returns. Cough and tiredness usually last for weeks after the rest of the illness is over.

The flu usually arrives in the winter months. 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. 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 died each year from complications of the flu.

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

  • People over 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 weakened immune system

Sometimes people confuse cold 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 several years.

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


  • Fever - may be high
  • Headache
  • Tiredness
  • Dry cough
  • Sore throat
  • Stuffy, congested nose
  • Muscle aches and stiffness

Other symptoms may include:

  • Nasal discharge
  • Shortness of breath
  • Wheezing
  • Croupy cough
  • Chills
  • Fatigue
  • Malaise
  • Sweating
  • Loss of appetite
  • Vomiting
  • Dizziness
  • Worsening of underlying illness, such as asthma or heart failure

Exams and Tests

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, 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 high risk for complications, antiviral medications may help shorten the length of symptoms by approximately a day.

In the past, doctors commonly prescribed the antiviral medications amantadine and  rimantadine to treat type A influenza. But in January 2006, the U.S. Centers for Disease Control and Prevention announced that neither drug should be used for such treatment because of growing resistance to the drugs. Instead, the agency recommended using oseltamivir (Tamiflu) or zanamivir (Relenza), which are active against both influenza A and B. Each of these medicines has different side effects and affects different viruses. Your doctor will determine which one is best for you.

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. Oseltamivir is the best choice for children. It is available in a liquid formulation and may be easier to give to the child than zanamivir, which is given by inhaler and is not FDA-approved for children under age 12. Treatment will only help if started early and only if the illness is actually influenza. It will not help treat a regular cold.

Outlook (Prognosis)

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

Possible Complications

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

When to Contact a Medical Professional

Call your health care provider if someone in a high-risk category develops symptoms of the flu.


A yearly vaccine is recommended for infants, the elderly, women who may be pregnant during flu season, and those with certain chronic health conditions. The vaccine is also recommended for people who work or live with others at high risk.

A flu shot can help lower one's chances of getting the flu. A flu shot is generally available to people who:

  • Are over 6 months old
  • Don't have a serious allergy to eggs
  • Haven't had a serious reaction to flu shots in the past
  • Are not pregnant

A new nasal spray-type flu vaccine called FluMist is available to people aged 5 to 49 years old. FluMist uses a live, weakened virus instead of a dead one like the flu shot. In one study, the nasal spray provided protection against the flu in up to 93% of children.

See also: Influenza vaccine.

CDC Recommends Against the Use of Amantadine and Rimantadine for the Treatment or Prophylaxis of Influenza in the United States during the 2005-06 Influenza Season. Atlanta, Ga. U.S. Centers for Disease Control and Prevention; January 14, 2006.

Long SS, Pickering LK, and Prober CG, eds. Principles and Practice of Pediatric Infectious Diseases. 2nd Ed. New York, NY: Churchill Livingstone, 2003:1159-1160.

Ferri FF. Ferri's Clinical Advisor: Instant Diagnosis and Treatment. 2005 ed. St. Louis, MO: Mosby; 2005:447-448.

Noble J., ed. Textbook of Primary Care Medicine. 3rd Ed. St. Louis, MO: Mosby; 2001.

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