Influenza (flu)

Date Updated: 11/01/2021

Overview

Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza is commonly called the flu, but it's not the same as stomach "flu" viruses that cause diarrhea and vomiting.

For most people, the flu resolves on its own. But sometimes, influenza and its complications can be deadly. People at higher risk of developing flu complications include:

Young children under age 5, and especially those under 6 months
Adults older than age 65
Residents of nursing homes and other long-term care facilities
Pregnant women and women up to two weeks after giving birth
People with weakened immune systems
Native Americans
People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes
People who are very obese, with a body mass index (BMI) of 40 or higher

Though the annual influenza vaccine isn't 100% effective, it's still your best defense against the flu.

Symptoms

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

Common signs and 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 doctor.

If you have flu symptoms and are at risk of complications, see your doctor right away. Taking antiviral drugs may reduce the length of your illness and help prevent more-serious problems.

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

Difficulty breathing
Blue lips
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 five 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 that 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 target children 6 months to 5 years old, and adults 65 years old or older.
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 drugs, 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 also increase the risk of developing complications.
Chronic illnesses. Chronic conditions, including lung diseases such as asthma, diabetes, heart disease, nervous system diseases, metabolic disorders, an airway abnormality, and kidney, liver or blood disease, may increase the risk of influenza complications.
Race. Native American 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 women are more likely to develop influenza complications, particularly in the second and third trimesters. Women are more likely to develop influenza-related complications up to two weeks after delivering their babies.
Obesity. People with a body mass index (BMI) of 40 or more 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 Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. The flu vaccine can reduce your risk of the flu and its severity and lower the risk of having serious illness from the flu and needing to stay in the hospital.

Flu vaccination is especially important this season because the flu and coronavirus disease 2019 (COVID-19) cause similar symptoms. Flu vaccination could reduce symptoms that might be confused with those caused by COVID-19. Preventing the flu and reducing the severity of flu illness and hospitalizations could also lessen the number of people needing to stay in the hospital.

It may be possible to get a COVID-19 vaccine at the same time as your flu vaccine.

This year's seasonal flu vaccine provides protection from the four influenza viruses that are expected to be the most common during the year's flu season. This year, the vaccine will be available as an injection and as a nasal spray.

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

Children under 2
Adults 50 and older
Pregnant women
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.

Both COVID-19 and the flu may be spreading at the same time. Your local health department and the CDC may suggest other precautions to reduce your risk of COVID-19 or the flu if you haven't been fully vaccinated. For example, you may need to practice social distancing (physical distancing) and stay at least 6 feet (2 meters) from others outside your household. You may also need to wear a cloth face mask when indoors with people outside your household and when outdoors in crowded areas. If you're fully vaccinated and are in an area with a high number of new COVID-19 cases in the last week, the CDC also recommends wearing a mask indoors in public and outdoors in crowded areas or when you're in close contact with unvaccinated people.

Diagnosis

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

During times when influenza is widespread, you may not need to be tested for influenza. Your doctor may diagnose you based on your signs and symptoms.

In some cases, your doctor may suggest that you be tested for influenza. He or she may use various tests to diagnose influenza. Polymerase chain reaction (PCR) testing is becoming more common in many hospitals and labs. This test may be done while you're in your doctor'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 influenza 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 doctor may prescribe an antiviral drug to treat the flu. These drugs can include oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) or baloxavir (Xofluza). These drugs may shorten your illness by a day or so and help prevent serious complications.

Oseltamivir is an oral medication. Zanamivir is inhaled through a device similar to an asthma inhaler and 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 drug is taken with food.

Most circulating strains of influenza have become resistant to amantadine and rimantadine (Flumadine), which are older antiviral drugs that are no longer recommended.

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 an over-the-counter pain reliever, such as 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.