Flu (Influenza)


What Is It?

Influenza, usually known as the flu, is a respiratory infection caused by one of the influenza viruses that typically is spread by air or by direct contact. Most cases occur during epidemics, which peak during the winter months nearly every year. A particularly widespread and severe epidemic is called a pandemic.

Compared with other viruses, influenza can strike remarkably large numbers of people in a relatively short time. Each year, about 25 million people seek medical care for the symptoms of influenza during flu season. In the United States, the Asian flu pandemic of 1957-1958 caused 70,000 deaths, and the Hong Kong flu of 1968-1969 killed 34,000 people. In the worst recorded pandemic of influenza, the 1918-1919 Spanish flu, 20 million people throughout the world died in less than one year.

The most common types of influenza virus are A and B. Influenza A is the one usually responsible for the annual epidemics. Most people get multiple influenza infections during their lives. With many other types of infections — for example, mumps — having the disease once protects against a second infection because the body’s immune system “remembers” the returning virus, attacks it immediately and rapidly eliminates it. With influenza, the virus usually has mutated (changed) somewhat since the first infection, but the change is enough to fool our immune system. So instead of immediately recognizing the invader, the immune system thinks it is a new infectious agent and the immune response is slowed. By the time the immune response is in full gear, millions of the body’s cells already have been infected with the virus.


Influenza can cause a variety of symptoms. They can be mild or severe depending upon the type of virus and your age and overall health. Although it is a respiratory virus, influenza can affect other body systems, making you feel sick all over. Symptoms can include any or all of the following:

  • Chills
  • Moderate to high fever (101 to 103 degrees Fahrenheit)
  • Sore throat
  • Runny nose
  • Muscle aches
  • Headaches
  • Fatigue
  • Cough
  • Diarrhea
  • Dizziness

Dangerous complications also can develop from influenza. One of the most feared complications, a bacterial “superinfection,” occurs when the influenza virus attacks the lung and weakens its defenses, allowing bacteria that normally live quietly in the nose and throat to descend into the lung and cause bacterial pneumonia. People over 50, infants, and those with certain chronic diseases or suppressed immune systems are especially vulnerable to complications.


Your doctor will evaluate your symptoms to determine whether you have influenza or a cold. If you are sneezing and have a stuffy nose, you may have a cold. Influenza is more likely to cause fever, coughing, chills and muscle aches. It also is more seasonal than colds. Flu tends to occur during winter months, whereas colds occur all year long.

Most of the time, doctors assume the diagnosis is influenza when you have an illness in the winter with the symptoms of influenza. If your symptoms or findings on physical examination suggest something other then the flu, your doctor may order blood tests and swab your nose and throat to send secretions to the laboratory for specific influenza testing. Your doctor may order a chest X-ray if he or she suspects that the influenza virus has caused a lung infection (pneumonia) or may lead to a bacterial superinfection and pneumonia.

Expected Duration

Influenza symptoms can last for as few as 24 hours, or for a week or more. A typical case lasts four or five days. As long as you have symptoms, you are contagious and can spread the infection to others.


Options for heading off an attack of influenza have increased steadily in recent years.

  • Vaccination — People who are at a high risk of infection with influenza, or are more likely to get complications if infected, should get vaccinated against the disease every year. Each year’s flu vaccine is different because it is adjusted to combat whatever virus strains are expected to be circulating that season. There is growing support for a policy of recommending the influenza vaccine to all adults, and many doctors will give the vaccine to any adult who requests it. But the vaccine is particularly recommended for people who:

    • Are 50 or older
    • Have certain chronic illnesses, especially those of the heart, lung or kidneys; diabetes; or severe forms of anemia
    • Have illnesses or are taking medications that reduce or suppress immune defenses
    • Live in nursing homes or assisted-living centers

    Others who should be vaccinated include:

    • Are pregnant women and will be in the second or third trimester during the epidemic
    • Are children ages 6 months to 23 months
    • Are children or teenagers who are receiving long-term aspirin therapy
    • Are health care professionals and others who are in close contact with anyone in the above high-risk groups

    The standard influenza vaccine is 70 percent to 90 percent effective in helping healthy people under age 65 to avoid the disease or lessen its severity. For maximum effectiveness, doctors advise people to get vaccinated in October or November, the start of the flu season.

    Healthy people between the ages of 5 and 49 have an alternative to the flu shot. FluMist is an intranasal vaccine spray that appears to offer similar protection. It uses a deactivated live virus rather than the killed virus in the shot. FluMist is not any more effective than the standard flu shot. Because FluMist is so new, people at the highest risk for influenza (people over age 49 and those with chronic health conditions) should receive the injected vaccine.

  • Good hygiene — The virus usually is passed through the air, by coughing and by direct contact, such as shaking hands or kissing. For this reason, practicing good hygiene — covering your mouth when you cough and washing your hands frequently — can help you to avoid getting the flu or spreading it to others.

  • Antiviral drugs — Two older antiviral drugs — amantadine (Symmetrel) and rimantadine (Flumadine) — and two newer ones — zanamivir (Relenza) and oseltamivir (Tamiflu) — can reduce your chance of getting the flu by 70 percent to 90 percent, if they are taken just before an expected outbreak. Zanamivir is a spray that is inhaled through the mouth. The others are pills. The older drugs, particularly amantadine, tend to have more side effects than the newer drugs. The side effects include nausea, vomiting, irritability, depression and difficulty sleeping. Zanamivir should generally be avoided in people who have asthma or chronic lung disease, since it may worsen wheezing. The two older drugs are effective only against the influenza type A virus, whereas the newer drugs also treat type B. The newer drugs are much more expensive.


To ease symptoms, your doctor will recommend that you rest and drink plenty of fluids (at least eight cups daily). For the fever and body aches, you can take over-the-counter pain relievers. If any of the four antiviral drugs mentioned in the Prevention section above are taken within 48 hours of the start of symptoms, they may speed recovery by about one day. Because influenza is a viral infection, antibiotics are not effective. Children who are suspected of having influenza, and who have high fevers, should never be given aspirin to treat the fever. This can cause the disease called Reye’s syndrome. Instead, acetaminophen (Tylenol) should be used.

When To Call A Professional

People with chronic diseases and sudden onset of classic flu symptoms should call their doctor’s office, because they might benefit from starting an antiviral medication within 48 hours of onset. Anyone with flu-like symptoms who experiences chest pain, ear pain, shortness of breath, fever that is not going away or a cough that produces blood or thick, foul-smelling mucus needs to notify his or her doctor.


Although most people recover fully from the flu, some develop serious complications, including life-threatening conditions such as pneumonia. About 20,000 people in the United States die of flu complications each year, and thousands more need to be hospitalized prior to recovery.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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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.