Influenza (flu)

Overview

Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Influenza viruses are different from the "stomach flu" viruses that cause diarrhea and vomiting. Most people with the flu get better on their own. But sometimes, influenza and its complications can be deadly.

Seasonal flu vaccines help prevent flu. Although the vaccine isn't 100% effective, it lowers the chances of having severe complications from the flu. This is especially true for people who are at high risk of flu complications.

Besides getting a vaccine, you can take other steps to help prevent respiratory illnesses, including flu. You can clean and disinfect surfaces, wash hands, and keep the air around you moving.

Symptoms

The viruses that cause flu spread at high levels during certain times of the year in the Northern and Southern hemispheres. These are called flu seasons. In the North, flu season is October to March, sometimes as late as May. In the South, flu season is usually between April and September.

Flu symptoms develop about 1 to 4 days after you're exposed to the virus. Symptoms usually start quickly.

Flu symptoms often include:

• Fever, but not always.
• Chills.
• Headache.
• Muscle pain.
• No appetite.
• Painful or watery eyes.

Other flu symptoms include:

• Cough.
• Sore throat.
• Runny or stuffy nose.
• Tiredness and weakness.
• Wheezing or shortness of breath.

While these symptoms are common, not everyone has them. In children, symptoms may show up more generally as being fussy or irritable. Children also are more likely than adults to have diarrhea, vomiting and an upset stomach.

When to see a doctor

Most people who get the flu can manage it at home and don't need to see a healthcare professional.

But if you have flu symptoms and are at risk of complications, see your healthcare professional right away. Antiviral medicine works best when you start it as soon as possible. That often means within two days after your symptoms show up. Taking medicine in that time frame may shorten the length of your illness.

If you have emergency symptoms of the flu, get medical care right away. For adults, emergency symptoms can include:

• Trouble breathing or shortness of breath.
• Chest pain or pressure.
• Ongoing dizziness.
• Trouble waking up or confusion.
• Dehydration.
• Seizures.
• Worsening of existing medical conditions.
• Severe weakness or muscle pain.

Emergency symptoms in children include all the symptoms seen in adults, as well as:

• Fast breathing or ribs that pull in with each breath.
• Gray or blue lips or nail beds.
• No tears when crying and a dry mouth, along with not needing to urinate.
• Symptoms, such as fever or cough, that get better but then come back or get worse.

Causes

Influenza is caused by viruses. These viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly. Or you can pick up the virus from touching an object, such as a computer keyboard, and then touching your eyes, nose or mouth.

It's possible to spread the virus to others from about a day before symptoms appear until about 5 to 7 days after they start. This is called being contagious. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing often. But the vaccines offered each year are made to match the flu virus strains that are most likely to spread that season. The protection these vaccines offer lasts for months in most people.

Risk factors

There is a range of factors that may raise your risk of catching a flu virus or having complications from a flu infection.

Treatments or medical conditions can raise the risk of getting flu or complications from flu, including:

• Weakened immune system. An immune system that doesn't quickly clear out flu virus may raise the risk of getting the flu or getting flu complications. A weakened immune system may be present from birth or may result from an illness, treatment or medicine.
• Chronic illnesses. Chronic conditions may increase the risk of influenza complications. Examples include asthma and other lung diseases, diabetes, heart disease, nervous system diseases, history of stroke, metabolic disorders, problems with the airway, and kidney, liver or blood disease.
• Aspirin therapy. Young people on long-term aspirin therapy are at risk of developing Reye's syndrome if infected with the influenza virus.

Other factors that can affect the risk of getting flu or getting seriously ill with flu are:

• Age. Seasonal influenza tends to have worse outcomes in young children, especially those age 2 years and younger. Adults older than age 65 also tend to have worse outcomes.
• Population group. In the United States, people who are Native American or Alaska Native, Black, or Latino may have a higher risk of needing care in the hospital for influenza.
• Living conditions. People who live in facilities with many other residents, such as nursing homes, are more likely to get the flu.
• Pregnancy. People who are pregnant are more likely to develop influenza complications, particularly in the second and third trimesters.
• Obesity. People with a body mass index (BMI) of 40 or higher have an increased risk of flu complications.

Complications

If you're young and healthy, the flu usually isn't serious. Although you may feel awful while you have it, the flu usually goes away in a week or two with no lasting effects. But people at high risk may develop other health problems after the flu, called complications.

Another infection can develop as a complication of the flu. That includes illnesses such as croup and sinus or ear infections. Lung infections are another complication. Infection of the heart muscle or heart lining may happen after getting the flu. And in some cases, people may have infection of the central nervous system.

Other complications may be:

• Acute respiratory distress syndrome.
• Muscle damage, called rhabdomyolysis, or muscle swelling, called myositis.
• Toxic shock syndrome.
• Worsening of a chronic illness, such as asthma or kidney disease.

Prevention

Seasonal flu vaccines are available in the U.S. for people age 6 months and older. Mayo Clinic recommends a seasonal flu vaccine for anyone who doesn't have a medical reason to avoid the vaccine.

Getting a flu vaccine lowers:

• The risk of getting the flu. If the vaccine is given later in pregnancy, the flu vaccine helps protect a newborn from the flu too.
• The risk of having serious illness from the flu and needing to stay in the hospital due to the flu.
• The risk of dying of the flu.

The 2026-2027 seasonal flu vaccines in the U.S. provide protection against three influenza viruses that researchers expect to be the most common this flu season.

The vaccine is available as a shot, a jet injector and a nasal spray.

If you have an egg allergy, you can still get a flu vaccine.

If a child is between ages 6 months and 8 years and has never had a flu vaccine or has had only one dose of a flu vaccine, the child may need two doses given at least four weeks apart. After that, the child can receive single annual doses of the flu vaccine. Check with your child's healthcare professional.

Also, check with your healthcare team before receiving a flu vaccine if you had a serious reaction to a previous flu vaccine. People who have had Guillain-Barre syndrome also should check with a healthcare professional before getting the flu vaccine. If you have a cold or don't feel well, you may still be able to get a flu shot. But if you are seriously ill, check with your healthcare team to see if you should delay getting the vaccine.

Seasonal flu shots

Flu shots are usually offered based on your age. For older children and adults, the flu shot is usually given in a muscle in the arm. Younger children may get the flu shot in a thigh muscle.

High-dose, recombinant or adjuvanted flu shots may be options. People over age 65 can get these vaccines. These vaccines also may be available to people age 18 and older who have a solid organ transplant and take medicine to weaken their immune response. These vaccines may help people in these groups avoid hospital care due to influenza.

People age 50 and older may have the option to get a flu shot that uses messenger RNA (mRNA) to protect against flu. This type of vaccine delivers instructions that tell the body how to make proteins from the outside of flu viruses but not the rest of the virus. For that reason, it can't give you a flu infection. The mRNA instructions are only for proteins on the surface of the flu virus. That's what the body sees first when you get an infection. After vaccination, your immune cells begin making the proteins and displaying them on cell surfaces. This causes your body to create antibodies that are ready to respond to a real infection.

Jet injector for seasonal flu shot

In the U.S., if you are an adult under the age of 65, you can choose to get your vaccine with a device called a jet injector. Instead of a needle, this device uses a high-pressure, narrow stream of fluid to give the vaccine through the skin.

Seasonal flu nasal spray

In the U.S., a seasonal flu nasal spray is approved for people between ages 2 and 49 years. If the nasal spray is an option for you or someone you care for, it may be available without an in-person visit with a healthcare professional.

The nasal spray flu vaccine isn't recommended for some groups, such as:

• People who have severe asthma or are currently wheezing.
• People who had a severe allergic reaction to a flu vaccine in the past.
• People who are pregnant.
• Young people who take aspirin or a medicine that has salicylate.
• People with severely weakened immune systems and caregivers or close contacts of people with severely weakened immune systems.
• Children ages 2 through 4 years old diagnosed with asthma or wheezing in the past 12 months.
• People who recently took antiviral medicine for the flu.
• People with a cerebrospinal fluid leak or the potential for a leak, such as those with a cochlear implant.

Check with your healthcare team to see if you need to be cautious about getting a nasal spray flu vaccine.

Controlling the spread of infection

You can take steps to lower the spread of infection, including:

• Wash your hands. Use soap and water if you can, and wash for at least 20 seconds. Or use a hand sanitizer with at least 60% alcohol.
• Don't touch your face. Try to keep your hands away from your eyes, nose and mouth. Those are places where germs can get into the body.
• Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Clean surfaces. Regularly clean often-touched surfaces to prevent spread of infection from touching a surface with the virus on it and then your face.

Also, try to lower the chance you'll come into contact with the flu virus. The flu spreads easily wherever people gather. When flu is spreading, try to keep space between yourself and others while indoors, especially in areas with poor air flow. Try to avoid people who are sick or have flu symptoms. And if you have symptoms, stay home if you can.