Influenza (patient information)

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What are the symptoms?

What are the causes?

Who is at highest risk?


When to seek urgent medical care?

Treatment options

Where to find medical care for Influenza?


What to expect (Outlook/Prognosis)?

Possible complications

Influenza On the Web

Ongoing Trials at Clinical

Images of Influenza

Videos on Influenza

FDA on Influenza

CDC on Influenza

Influenza in the news

Blogs on Influenza

Directions to Hospitals Treating Influenza

Risk calculators and risk factors for Influenza

For the WikiDoc page for this topic, click here For patient information about non-human influenza types that may infect humans, see Avian influenza (bird flu) or Swine influenza (pig flu)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Assistant Editor-in-Chief: Alexandra M. Palmer


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

What are the symptoms of Influenza?

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:

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:

What causes Influenza?

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

How Flu Spreads

Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouth or noses of people who are nearby. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth, eyes or possibly their nose.

Symptoms of Flu

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.

Who is at highest risk?

Since the flu is contagious, anyone around a person with the flu is at risk.


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.

When to seek urgent medical care?

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

Treatment options

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.

Where to find medical care for Influenza?

Directions to Hospitals Treating Influenza


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 detailed information about the nasal spray flu vaccine, click here

Flu shots

Who should get a flu shot?

Get a flu shot every year if you are age 50 or older. For many people, the flu is a mild illness.

When should I get the flu shot?

  • Yearly flu vaccination should begin soon after flu vaccine is available, and ideally by October.
  • Getting vaccinated even later can be protective, as long as flu viruses are circulating.
  • Seasonal influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later. Since it takes about two weeks after vaccination for antibodies to develop in the body that protect against influenza virus infection, it is best that people get vaccinated so they are protected before influenza begins spreading in their community.

Are there any side effects?

Side effects from the flu shot are mild. Some people feel sore at the spot where they got the shot. There is no reason to worry. You cannot get the flu from the flu shot. The flu shot is made from dead flu virus that will not cause the flu.

Spray intranasal influenza vaccine

For detailed information about the nasal spray flu vaccine, click here

What to expect (Outlook/Prognosis)?

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

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:

The complications of influenza can be seen in older adults with:

What health problems can the flu make worse?

The flu can also make some health problems worse. These health problems include:

Sources Template:WH Template:WS