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Flu (influenza)

Medically reviewed by Last updated on Apr 12, 2023.

What is the Flu (influenza)?

Harvard Health Publishing

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, up to 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:

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:


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.

For maximum effectiveness, doctors advise getting vaccinated at the start of flu season. This generally means October or November. There are a number of different types of vaccinations available to prevent influenza. They vary by how they are given (for example, by injection or inhaled through the nose), by the type and number of virus strains covered and by the method of vaccination production (for example, using an altered live virus or an inactivated virus).

Recommendations vary for different people and from year-to-year so ask your doctor which specific vaccination is best for you.

Other ways to protect yourself from getting the flu include:

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.


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 baloxavir (Xofluza), oseltamivir (Tamiflu), peramivir (Rapivab) or zanamivir (Relenza) are also options, depending on how long you've been sick (since treatment may speed recovery only if started soon after the onset of symptoms), severity of symptoms and risk factors for complications. In general, starting one of these medications three or more days after the onset of flu symptoms is unlikely to be effective.

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 a serious illness called Reye's syndrome.

Treatment options

The following list of medications are related to or used in the treatment of this condition.

View more treatment options

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


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

Additional info

Centers for Disease Control and Prevention (CDC)

Further information

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