Influenza (flu)

Overview

Flu (influenza) is an infection of the nose, throat and lungs, which are part of the respiratory system. Influenza is commonly called the flu, but it's not the same as 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. People at higher risk of developing flu complications include:

  • Young children under age 2
  • Adults older than age 65
  • Residents of nursing homes and other long-term care facilities
  • People who are pregnant or plan to be pregnant during flu season
  • People with weakened immune systems
  • American Indians or Alaska Natives
  • People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes
  • People with a body mass index (BMI) of 40 or higher

Although the annual influenza vaccine isn't 100% effective, it reduces the chances of having severe complications from infection.

Symptoms

At first, the flu may seem like a common cold with a runny nose, sneezing and sore throat. Colds usually develop slowly. But the flu tends to come on suddenly. And while a cold can be miserable, you usually feel much worse with the flu.

Common symptoms of the flu include:

  • Fever
  • Aching muscles
  • Chills and sweats
  • Headache
  • Dry, persistent cough
  • Shortness of breath
  • Tiredness and weakness
  • Runny or stuffy nose
  • Sore throat
  • Eye pain
  • Vomiting and diarrhea, but this is more common in children than adults

When to see a doctor

Most people who get the flu can treat themselves at home and often don't need to see a health care provider.

If you have flu symptoms and are at risk of complications, see your health care provider right away. Taking antiviral medication may shorten the length of your illness and help prevent more-serious problems.

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

  • Difficulty breathing or shortness of breath
  • Chest pain
  • Ongoing dizziness
  • Seizures
  • Worsening of existing medical conditions
  • Severe weakness or muscle pain

Emergency symptoms in children can include:

  • Difficulty breathing
  • Pale, gray or blue-colored skin, lips or nail beds — depending on skin color
  • Chest pain
  • Dehydration
  • Severe muscle pain
  • Seizures
  • Worsening of existing medical conditions

Causes

Influenza 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 germs from an object — such as a telephone or computer keyboard — and then transfer them to your eyes, nose or mouth.

People with the virus are likely contagious from about a day before symptoms appear until about four days after they start. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing regularly. If you've had influenza in the past, your body has already made antibodies to fight that specific strain of the virus. If future influenza viruses are similar to those you've encountered before, either by having the disease or by getting vaccinated, those antibodies may prevent infection or lessen its severity. But antibody levels may decline over time.

Also, antibodies against influenza viruses you've encountered in the past may not protect you from new influenza strains. New strains can be very different viruses from what you had before.

Risk factors

Factors that may increase your risk of developing the flu or its complications include:

  • Age. Seasonal influenza tends to have worse outcomes in children under age 2, and adults older than age 65.
  • Living or working conditions. People who live or work in facilities with many other residents, such as nursing homes or military barracks, are more likely to develop the flu. People who are staying in the hospital also are at higher risk.
  • Weakened immune system. Cancer treatments, anti-rejection medications, long-term use of steroids, organ transplant, blood cancer or HIV/AIDS can weaken the immune system. This can make it easier to catch the flu and may increase the risk of developing complications.
  • Chronic illnesses. Chronic conditions may increase the risk of influenza complications. Examples include asthma and other lung diseases, diabetes, heart disease, nervous system diseases, metabolic disorders, problems with an airway, and kidney, liver or blood disease.
  • Race. American Indians or Alaska Natives people may have an increased risk of influenza complications.
  • Aspirin use under age 19. People who are younger than 19 years of age and receiving long-term aspirin therapy are at risk of developing Reye's syndrome if infected with influenza.
  • Pregnancy. Pregnant people are more likely to develop influenza complications, particularly in the second and third trimesters. This risk continues up to two weeks after the baby is born.
  • 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 miserable while you have it, the flu usually goes away in a week or two with no lasting effects. But children and adults at high risk may develop complications that may include:

  • Pneumonia
  • Bronchitis
  • Asthma flare-ups
  • Heart problems
  • Ear infections
  • Acute respiratory distress syndrome

Pneumonia is one of the most serious complications. For older adults and people with a chronic illness, pneumonia can be deadly.

Prevention

The U.S. Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. The flu vaccine can lower your risk of getting the flu. It also can lower the risk of having serious illness from the flu and needing to stay in the hospital.

Flu vaccination is especially important because the flu and coronavirus disease 2019 (COVID-19) cause similar symptoms. Both COVID-19 and the flu may be spreading at the same time. Vaccination is the best way to protect against both. Flu vaccination could lessen symptoms that might be confused with those caused by COVID-19. Preventing the flu and lowering the number of people with severe flu and complications could also lower the number of people needing to stay in the hospital. And if a COVID-19 vaccine or booster and a flu vaccination are due at the same time, the CDC reports that you can get vaccinated for both in one visit.

This year's seasonal flu shot provides protection against four influenza viruses expected to be the most common during this flu season. This year, the vaccine will be available as an injection and as a nasal spray. There will also be a high-dose flu vaccine for adults age 65 and older.

The nasal spray is approved for people between 2 and 49 years old. It isn't recommended for some groups, such as:

  • Children younger than age 2
  • Adults age 50 and older
  • Pregnant people
  • Children between 2 and 17 years old who are taking aspirin or a salicylate-containing medication
  • People with weakened immune systems
  • Kids 2 to 4 years old who have had asthma or wheezing in the past 12 months

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

Controlling the spread of infection

The influenza vaccine isn't 100% effective, so it's also important to take several measures to reduce the spread of infection, including:

  • Wash your hands. Washing your hands often with soap and water for at least 20 seconds is an effective way to prevent many common infections. Or use alcohol-based hand sanitizers if soap and water aren't available.
  • Avoid touching your face. Avoid touching your eyes, nose and mouth.
  • 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.
  • Avoid crowds. The flu spreads easily wherever people gather — in child care centers, schools, office buildings, auditoriums and public transportation. By avoiding crowds during peak flu season, you reduce your chances of infection.

Also avoid anyone who is sick. And if you're sick, stay home for at least 24 hours after your fever is gone so that you lessen your chance of infecting others.

Diagnosis

Your health care provider will conduct a physical exam, look for signs and symptoms of flu, and possibly order a test that detects influenza viruses.

During times when flu is widespread, you may not need to be tested for it. Your health care provider may diagnose you based on your symptoms.

In some cases, your health care provider may suggest that you be tested for influenza. Your provider may use many tests to diagnose flu. Polymerase chain reaction (PCR) testing is becoming more common in many hospitals and labs. This test may be done while you're in your health care provider's office or in the hospital. PCR testing is more sensitive than other tests and may be able to identify the influenza strain.

It's possible to have a test to diagnose both flu and COVID-19. You may have both COVID-19 and influenza at the same time.

Treatment

Usually, you'll need nothing more than rest and plenty of fluids to treat the flu. But if you have a severe infection or are at higher risk of complications, your health care provider may prescribe an antiviral medication to treat the flu. These drugs can include oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) or baloxavir (Xofluza). These medications may shorten your illness by a day or so and help prevent serious complications.

Oseltamivir is a medicine taken by mouth. Zanamivir is inhaled through a device similar to an asthma inhaler. It shouldn't be used by anyone with certain chronic respiratory problems, such as asthma and lung disease.

Antiviral medication side effects may include nausea and vomiting. These side effects may be lessened if the medication is taken with food.

Lifestyle and home remedies

If you do come down with the flu, these measures may help ease your symptoms:

  • Drink plenty of liquids. Choose water, juice and warm soups to prevent dehydration.
  • Rest. Get more sleep to help your immune system fight infection. You may need to change your activity level, depending on your symptoms.
  • Consider pain relievers. Use acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others), to combat the achiness associated with influenza. Children and teens recovering from flu-like symptoms should never take aspirin because of the risk of Reye's syndrome, a rare but potentially fatal condition.

To help control the spread of influenza in your community, stay home and keep sick children home until the fever has been gone for 24 hours. Avoid being around other people until you're feeling better, unless you're getting medical care. If you do need to leave your home and get medical care, wear a face mask. Wash your hands often.


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