What is influenza (flu)?

Updated for the 2021-22 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 during 2021-2022 is more important than ever because of the ongoing COVID-19 pandemic. Flu vaccines and COVID-19 vaccines can be given at the same time. 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. Lab tests may be used to confirm the diagnosis, if needed.

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. Bed rest and plenty of fluids can help.

Talk with your healthcare provider for more information.

What are possible complications of the flu?

The most common complication of the flu is pneumonia. It can also cause ear and sinus infections. In rare cases, it may cause serious muscle, heart, and central nervous system problems. Of those who get the flu, CDC estimates that 140,000 to 810,000 end up in the hospital each year. From 12,000 to 61,000 people have died from the flu each year since 2010. Most of these deaths happen in people ages 65 and older or in those with other health problems such as diabetes, asthma, heart disease, cancer, or HIV/AIDS.

Can the flu be prevented?

A new flu vaccine is made each fall to protect against the flu viruses predicted to cause outbreaks during that flu season. It's one of the best ways to protect yourself. Everyone ages 6 months and older should get a flu shot each year. It's usually recommended for specific groups of people, as well as for anyone who doesn't want to get the flu. For the 2021-2022 influenza season, 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. Because of the COVID-19 pandemic, health experts strongly advise that you get the flu vaccine to protect you and others, and to reduce the strain on the healthcare system because of COVID-19. The flu shot is also recommended due to the possibility that getting COVID-19 and the flu at the same time could be more deadly than having either infection alone.

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 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 till 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.

The flu vaccine is recommended even for people with egg allergies. People with a history of severe egg allergies are advised to get their vaccine in a medical setting such as a hospital or outpatient medical clinic overseen by a healthcare provider who can recognize and manage severe allergic reactions. For people age 18 or older, an egg-free vaccine may be available, but it is generally not needed. Talk with your healthcare provider about which flu vaccine is right for you and where you should get your vaccine.

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 bed 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 the ongoing COVID-19 pandemic.

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.