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Flu (Influenza)

Medically reviewed by Last updated on Aug 27, 2018.

What Is It?

Harvard Health Publications

Influenza (the flu) is a respiratory infection. It is caused by the influenza virus. Influenza typically is spread by air or by direct contact from one person to another. Influenza virus is very contagious.

Most influenza cases occur during epidemics. Epidemics usually peak during the winter months. 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. In the developed nations, about 10-15% of the people get the flu each year. During severe epidemics, a greater fraction of the population gets sick.

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 flu infections during their lives. With many other types of infections, having the disease once protects against a second infection. That is because the body's immune system remembers the returning virus. It attacks it immediately, and rapidly eliminates it.

With influenza, the virus usually has mutated (changed) somewhat since the first infection. The change is enough to fool your immune system. As a result, the immune system responds slowly. By the time the immune response is in full gear, millions of the body's cells are already infected.


Flu can cause a variety of symptoms. They can be mild or severe. Symptoms and illness severity depend on the type of virus, your age and overall health.

Although it is a respiratory virus, flu can affect other body systems. This makes you feel sick all over. Symptoms can include any or all of the following:

  • Chills

  • Moderate to high fever (101 to 103 degrees Fahrenheit)

  • Muscle aches

  • Headaches

  • Fatigue

  • Cough

  • Sore throat

  • Runny nose

  • Diarrhea

  • Dizziness

Dangerous complications, such as pneumonia, can develop from flu. The influenza virus can be the direct cause of pneumonia. But also infection with influenza makes a person more susceptible to bacterial pneumonia.

Certain people are especially vulnerable to complications. These include:

  • Older people

  • Infants

  • People with certain chronic diseases

  • People with suppressed immune systems


Your doctor will evaluate your symptoms. Flu is likely to cause fever, coughing, chills and muscle aches. Flu tends to occur during winter months.

Doctors usually assume the diagnosis is flu when you have symptoms of influenza in the winter. If your symptoms or physical examination suggest something other than the flu, your doctor may order a blood test. He or she will swab your nose and throat for influenza testing.

Your doctor may order a chest X-ray. This is likely if he or she suspects that the influenza virus has caused pneumonia or may lead to a bacterial superinfection.

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.


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

  • Vaccination — Vaccination can reduce your chances of getting the flu and transmitting it to others. Vaccination each year is recommended for everyone aged 6 months and older.

Vaccination is particularly recommended for:

  • All children and adolescents aged 6 months to 18 years of age. This is especially true for those receiving long-term aspirin therapy. That is because children taking aspirin are at increased risk for experiencing a serious illness called Reye's syndrome if they get influenza infection.

  • All people older than 50 years

  • Women who are pregnant or will be pregnant during the influenza season

  • Adults and children who have disorders that affect their:

    • Lungs, including chronic lung disorders such as asthma

    • Heart

    • Kidney

    • Liver

    • Blood

    • Metabolism (including diabetes)

  • Adults and children whose immune systems are suppressed

  • Adults and children who have any condition that can:

    • Compromise lung function

    • Compromise the handling of respiratory secretions

    • Increase the risk for aspiration such as mental impairment, spinal cord injuries, seizure disorders, or other neuromuscular disorders.

  • Residents of nursing homes and other chronic-care facilities

  • Health-care personnel

  • Adults or children who are in close contact with:

    • Children younger than 5 years (especially children younger than 6 months)

    • Adults older than 50 years

  • Adults or children who are in close contact with people who have medical conditions that put them at higher risk for severe complications from influenza

For maximum effectiveness, doctors advise getting vaccinated at the start of flu season. This generally means October or November.

Other ways to protect yourself from getting the flu:

  • FluMist — Healthy people between the ages of 2 and 49 have an alternative to the flu shot. FluMist is an intranasal vaccine spray. It appears to offer similar protection to the flu shot. FluMist uses a deactivated live virus rather than the killed virus in the shot. FluMist is not any more effective than the standard flu shot.

People at the highest risk for flu should still receive the injected vaccine. This includes people older than 49 and those with chronic health conditions.

  • Good hygiene — The virus usually is passed through the air, by coughing. It also is passed by direct contact, such as shaking hands or kissing.

Practicing good hygiene can help you to avoid getting the flu or spreading it to others. Good hygiene includes covering your mouth when you cough and washing your hands frequently.

  • Antiviral drugsZanamivir (Relenza) and oseltamivir (Tamiflu) can substantially reduce your chance of getting the flu if they are taken just before an expected outbreak.

Zanamivir is given by inhalation from a nebulizer. It is approved for prevention in people ages 5 and older and for treatment in people ages 7 and older.

Oseltamivir is available in tablet form. It is approved for prevention and treatment in patients older than one year.


To ease symptoms, your doctor will recommend that you rest and drink plenty of fluids.

For fever and body aches, you can take over-the-counter pain relievers. The antiviral drugs zanamivir, oseltamivir or peramivir are another option.

An anti-viral drug may speed recovery by about one day. It needs to be started within 48 hours of the start of symptoms. The options are oral oseltamivir (Tamiflu), inhaled zanamivir (Relenza) or peramivir (Rapivab) given intravenously (by IV) in the doctor's office.

Because flu is a viral infection, antibiotics are not effective.

Children who are suspected of having flu, and who have high fevers should be given acetaminophen (Tylenol). They should never be given aspirin to treat the fever. This can cause the disease called Reye's syndrome.

When To Call a Professional

If you have a chronic disease and suddenly get flu symptoms, call your doctor's office. You may benefit from starting an antiviral medication within 48 hours.

You also should notify your doctor if you have flulike symptoms along with:

  • Chest pain

  • Ear pain

  • Shortness of breath

  • Fever that does not go away

  • A cough that produces blood or thick, foul-smelling mucus


Most people recover fully from the flu. But some develop serious complications. Complications can include life-threatening conditions such as pneumonia.

Learn more about Flu (Influenza)

Associated drugs

IBM Watson Micromedex

External resources

Centers for Disease Control and Prevention (CDC)
1600 Clifton Road
Atlanta, GA 30333
Phone: 404-639-3534
Toll-Free: 1-800-311-3435

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.