What is influenza (flu)?

Updated for the 2024-2025 influenza season

Influenza (flu) is an easily spread respiratory tract infection. It's caused by a virus. Millions of people get the flu each year. The flu usually starts abruptly, with fever, muscle aches, sore throat, and a cough.

The flu can make people of any age sick. Most people are sick with the flu for only a few days. But some have a much more serious illness. They may need to go to the hospital. The flu can also lead to pneumonia and death.

The flu viruses continually change. Vaccines are developed and given each year to protect against the flu virus strains expected to cause the illness that year.

Getting a flu vaccine is more important than ever because of other illnesses such as COVID-19 and RSV. Flu vaccines are especially important for people who are at high risk from complications of the flu.

What causes the flu?

The flu is caused by a virus. Viruses are generally passed from person to person through the air when an infected person sneezes or coughs.

But the virus can also live for a short time on objects like doorknobs, pens, pencils, keyboards, phones, and cups or eating utensils. So you can also get the flu by touching something that has been recently handled by someone infected with the virus and then touching your own mouth, nose, or eyes.

What are the symptoms of the flu?

Each person may have different symptoms. The flu is a respiratory disease. But it can affect your whole body. Symptoms usually start suddenly. People usually become very sick with several, or all, of these symptoms:

• Cough, often becoming severe

• Extreme exhaustion

• Fatigue for several weeks

• Headache

• High fever

• Runny or stuffy nose

• Severe aches and pains

• Sneezing at times

• Sometimes a sore throat

• Vomiting and diarrhea

Fever and body aches often last for 3 to 7 days. But cough and fatigue may last for 2 weeks or more.

The symptoms of the flu may look like other health problems. Always talk with your healthcare provider for a diagnosis.

How is the flu diagnosed?

The flu is diagnosed based on your symptoms and lab tests. Lab tests involve a swab of your nose to confirm the diagnosis.

How is the flu treated?

Treatment will depend on your symptoms, age, and general health. It will also depend on how severe the condition is.

The goal of treatment for the flu is to help prevent or decrease the severity of symptoms and any complications. Treatment may include:

• Antiviral medicines. These prescription medicines can reduce how long you’ll have the flu. In some high-risk people, they can also lower the risk of complications or shorten how long they last. These medicines generally have to be started within the first 2 days of the illness. But people at the highest risk for complications or those who already have them may be given the medicines even after the second day of being sick. These medicines do sometimes have side effects, such as nervousness, lightheadedness, or nausea. But they are often mild at most.

• Medicines. There are over-the-counter medicines for congestion and nasal discharge. You can also take medicine to relieve aches and fever. Don't give aspirin to children or teens with fever. Aspirin may cause side effects, such as an upset stomach and intestinal bleeding. It can also cause Reye syndrome. This rare but very serious illness can affect all organs of the body. But it most often injures the brain and liver. The medicine of choice for children and teens is acetaminophen.

• Rest. Get plenty of rest.

• Staying hydrated. Drink fluids like water or broth.

Talk with your healthcare provider for more information.

What are possible complications of the flu?

The flu can cause ear and sinus infections. Flu can make chronic conditions worse. For example, it can cause flares in people with asthma.

A serious complication of the flu is pneumonia. Other possible serious complications include infection and inflammation of the heart (myocarditis), brain (encephalitis), or muscles (myositis or rhabdomyolysis). Flu can sometimes cause a life-threatening inflammation in the body called sepsis. Sepsis is a medical emergency.

Can the flu be prevented?

Health experts strongly advise that you get the flu vaccine every year to protect yourself and others. A new flu vaccine is made each fall to protect against the flu viruses predicted to cause outbreaks during that flu season. Everyone age 6 months or older should, with rare exceptions, get a flu shot each year. The vaccine is available in different forms. The most common way to get the vaccine is by flu shot. A nasal spray is also available for healthy, nonpregnant people between ages 2 and 49.

The flu shot is safe. The CDC and the FDA closely watch vaccine safety. Hundreds of millions of flu vaccines have been safely given across the country for decades.

The flu shot can’t give you the flu. But some of the side effects can be like the illness. The most common side effects from a flu shot are:

• Achiness

• Low-grade fever

• Soreness where the shot was given

If you have them at all, these side effects are usually mild and last a short time.

The effectiveness of the vaccine varies from one person to another. It can depend on factors such as age and overall health.

The following may also be helpful for preventing the flu:

• When possible, stay away from or limit contact with sick people.

• Wash your hands frequently with soap and water to reduce the risk of infection.

• Cover your nose and mouth when coughing or sneezing to limit the spread of the virus.

The flu causes complications that may develop into a more serious disease or become dangerous to some people. This includes older adults and those with long-term (chronic) health problems. Always talk with your healthcare provider to find out if you should get the flu shot.

Although the flu shot is safe, some people should not be vaccinated. These include:

• People who have had a severe, life-threatening reaction in the past after getting the flu shot

• Babies age 6 months old or younger

Talk with your healthcare provider before getting a flu shot if:

• You are sick with a fever. Talk with your provider first. You may be advised to wait until you recover to get the shot.

• Have had a severe paralyzing illness called Guillain-Barré syndrome (GBS) after getting a flu shot in the past. Some people with a history of GBS should not get a flu shot.

Most flu shots and the nasal spray flu vaccine are made using egg-based technology. The flu vaccine (either egg-based or non-egg-based) is recommended even for people with egg allergies. All vaccines should be given in settings where allergic reactions can be recognized and treated quickly. Talk with your healthcare provider about your risk. Ask which flu vaccine is right for you.

September and October are generally good times to be vaccinated. The CDC recommends getting the flu shot every year by the end of October. Flu season can start as early as October and most commonly peaks in the U.S. in January or February. But flu seasons are unpredictable. The flu shot takes 1 to 2 weeks to start working. Even if you can't get your vaccine by the end of October, experts still recommend getting it as long as the flu season is active.

The CDC recommends that travelers have the flu vaccine at least 2 weeks before planned travel to allow time to develop immunity. Talk with your healthcare provider for more information.

When should I call my healthcare provider?

For most people, the flu can be treated at home without treatment from your healthcare provider. But if you have other health problems that make you more susceptible to complications from the flu, tell your healthcare provider when you suspect you have the flu. If your symptoms get worse or you have new symptoms, let your provider know.

Key points about the flu

• The flu is an easily spread viral respiratory tract infection.

• The flu is caused by viruses that are generally passed from person to person through the air.

• The flu is treated with rest, plenty of fluids, and medicines to treat discomfort and fever.

• Antiviral medicines taken within the first 2 days of illness can reduce the length and severity of the disease. They may also reduce the risk of complications in those at high risk.

• Getting the flu vaccine every year is the best prevention. Flu vaccines are more important than ever because of other illnesses such as COVID-19 and RSV.

Next steps

Tips to help you get the most from a visit to your healthcare provider:

• Know the reason for your visit and what you want to happen.

• Before your visit, write down questions you want answered.

• Bring someone with you to help you ask questions and remember what your healthcare provider tells you.

• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.

• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are.

• Ask if your condition can be treated in other ways.

• Know why a test or procedure is recommended and what the results could mean.

• Know what to expect if you do not take the medicine or have the test or procedure.

• If you have a follow-up appointment, write down the date, time, and purpose for that visit.

• Know how you can contact your healthcare provider if you have questions, especially after office hours and on weekends and holidays.