The flu

Alternative names
Flu; Influenza A; Influenza B; Influenza C

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

Causes, incidence, and risk factors

The flu usually begins abruptly, with a fever between 102 to 106°F (with adults on the lower end of the spectrum). 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 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 (red) throat and a headache. Nasal discharge and sneezing are common. These symptoms (except the cough) usually disappear within 4-7 days. Sometimes there’s a second wave of fever at this time. Cough and tiredness usually last for weeks after the rest of the illness is over.

The most common way to catch the flu is by inhaling 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 with 2-3 days). Because the flu is airborne and very contagious, with a short incubation period, it often slams into a community all at once. This creates a cluster of school and work absences. The flu usually arrives in the winter months. 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 114,000 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 occur at the same time of the year (cold and flu season). 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. Influenza infections are primarily respiratory infections, and may have other symptoms as well.


  • 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

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

Additional blood work may be warranted during the evaluation, which may include a complete blood count, blood cultures, and sputum cultures.

The most common methods for diagnosing influenza include antigen detection tests, which are 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, but the diagnosis can often be made by 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 influenza is diagnosed within 48 hours of when symptoms begin, especially if you are high risk for complications, several antiviral medications are available that may shorten the duration of symptoms by approximately 1 day. These include amantadine or rimantadine (active against influenza A only); oseltamivir and zanamivir (active against influenza A and B). Each of these medicines has different side effects and affects different viruses. Your physician 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 (Tamiflu) is the best choice for children. It is available in a liquid formulation and may be easier to give to the child than zanamivir (Relenza), which is not licensed for children less than 12 years old and needs to be given by via 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)

In most individuals who are otherwise healthy, influenza fully resolves 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

Calling your health care provider

Call your health care provider if someone in a high-risk category develops symptoms of the flu. For healthy, non-pregnant people between 2 and 49 years old, see the articles on fever, cough, vomiting, and other specific symptoms about when it is important to call a health care provider.


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 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 or institution  
  • Anyone with chronic heart, lung, or kidney conditions, diabetes, or weakened immune system

Anyone can get the flu shot if they want to lower their chances of getting the flu, as long as they:

  • 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

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

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.

Johns Hopkins patient information

Last revised: December 2, 2012
by Arthur A. Poghosian, M.D.

Medical Encyclopedia

  A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 0-9

All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.