The flu

• Illustrations
• Alternative names
• Definition
• Causes, incidence, and risk factors
• Symptoms
• Signs and tests

• Treatment
• Expectations (prognosis)
• Complications
• Calling your health care provider
• Prevention
• References

Alternative names    Return to top

Definition    Return to top

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

Causes, incidence, and risk factors    Return to top

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 is airborne and very contagious, with a short incubation period, it often strikes a community all at once. This creates a cluster of school and work absences. Within 2 or 3 weeks of its arrival in a school, much of the classroom has had it.

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

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

• People over 50 years of age
• Children 6-23 months
• 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 something of a myth, as the stomach symptoms are not caused by the flu virus. Flu infections are primarily respiratory infections.

Symptoms    Return to top

• 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

Signs and tests    Return to top

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.

Treatment    Return to top

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 1 day. Medications to treat influenza A include amantadine or rimantadine. Oseltamivir (Tamiflu) and zanamivir (Relenza) 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 not approved for children under age 12. Zanamivir needs to be given by by inhaler. Treatment will only help if started early and only if the illness is actually influenza. It will not help treat a regular cold.

Expectations (prognosis)    Return to top

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

Complications    Return to top

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

• pneumonia
• encephalitis (infection of the brain)
• bronchitis
• sinus infections
• ear infections

Calling your health care provider    Return to top

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

Prevention    Return to top

A yearly flu vaccine is recommended for those people in high-risk categories. The vaccine is also recommended for people who work with (or live with) others at high risk.

People at high risk for the flu include:

• People over 50 years of age
• Children aged 6 to 23 months
• 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

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

The FluMist nasal spray is an alternative. Unlike the regular vaccine, it is a live virus. Therefore, it is best if the person receiving it does not have close contact with people with a weakened immune system.

Some people, particularly those at high risk, who have been exposed to the flu virus may benefit from antiviral medication and vaccination.

References    Return to top

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.