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.

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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

Related information

COVID-19 and flu: Similarities and differences

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 are also 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 your immune system. This can make it easier for you to catch the flu and may also increase your 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 your 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.

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

In recent years, there was concern that the nasal spray vaccine wasn't effective enough against certain types of flu. However, the nasal spray vaccine is expected to be effective in the 2020-2021 season. The nasal spray still isn't recommended for some groups, such as pregnant women, children between 2 and 4 years old with asthma or wheezing, and people who have compromised immune systems.

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

Mayo Clinic Minute: Why you need your flu shot now

Vivien Williams: Seasonal flu is a virus that attacks your respiratory system. Most people get better on their own, but, for some, the flu can be serious.

Tina Ardon, M.D.: Our babies, our younger children and our older adults definitely can get more sick from the flu. Patients who are already sick with other chronic conditions, are undergoing chemotherapy are more likely to have complications from the flu as well.

Vivien Williams: Complications include bronchitis, pneumonia and even heart problems.

Dr. Ardon: The vaccine for influenza is one our best defenses we have.

Vivien Williams: Dr. Tina Ardon says some people don't get the flu vaccine because they think it could cause the flu.

Dr. Ardon: The flu vaccine is what we call a dead vaccine. So you cannot get the actual flu from the flu vaccine.

Vivien Williams: You can't get the flu from a flu shot. FluMist, while safe for most people, contains a weakened form of the virus, so people with certain conditions should get a shot instead.

Dr. Ardon: Everyone who can get it should get it.

Vivien Williams: It takes two weeks for the vaccine to work, so get your flu shot now. For the Mayo Clinic News Network, I'm Vivien Williams.

Mayo Clinic Minute: The facts about three flu vaccine myths

Ian Roth: Influenza myth No. 1: The flu is like a cold. It will make you feel awful, but it isn't really harmful. Fact.

Gregory Poland, M.D.: Well, in the U.S. alone, last year 80,000 Americans died from influenza and its complications, and almost a million were hospitalized.

Ian Roth: The complications are the really dangerous part. Dr. Gregory Poland says influenza commonly leads to deadly pneumonia, meningitis, heart attacks, and complications for people with diabetes.

Ian Roth: Myth No. 2: I always get the flu from the flu shot. Fact.

Dr. Poland: It is impossible to get influenza from the vaccine.

Ian Roth: Dr. Poland says it's not uncommon for people to coincidentally get an upper respiratory infection in the weeks after getting a flu shot and assume it's connected. But he says studies have proven your odds of getting that minor upper respiratory infection from other viruses are identical whether you get the flu shot or not, so you might as well protect yourself from the more serious influenza.

Ian Roth: And myth No. 3: Science still doesn't know enough about vaccines to know if they're safe. Fact.

Ian Roth: Dr. Poland says vaccines are among the safest and most studied areas of modern medicine. In fact, it takes more study and research to get a vaccine approved than it does for an over-the-counter headache medicine like acetaminophen or ibuprofen. For the Mayo clinic News Network, I'm Ian Roth.

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.

During the COVID-19 pandemic, 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. 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 around people outside your household.

Sept. 19, 2020
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