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Influenza (Seasonal Flu)

Condition Basics

What is influenza (flu)?

Influenza (flu) is an infection from a virus. It can cause a fever, body aches, a headache, and a cough. It is contagious and usually lasts about a week or two. Flu symptoms are usually worse than a cold and last longer. The flu usually doesn't cause vomiting or diarrhea in adults.

Most flu outbreaks happen in late fall and winter. Because symptoms may not start for a couple of days, you may pass the flu to someone before you know you have it.

What causes it?

The flu is usually caused by the influenza viruses types A and B. There are different subtypes, or strains, of the flu virus every year.

What are the symptoms?

Flu symptoms may include fever, body aches, a headache, a dry cough, and a sore or dry throat. You'll probably feel tired and less hungry than usual. The symptoms are usually worse for the first few days. But it can take up to a few weeks to get completely better.

How is it diagnosed?

Your doctor will ask you about your symptoms and will examine you. This usually gives the doctor enough information to find out if you have influenza (flu). Sometimes the doctor will do a test to check for the flu.

How is the flu treated?

Most people can treat influenza (flu) symptoms at home with rest, medicines, or other treatment. Your doctor may give you medicine that can make the symptoms milder. But some people need treatment in the hospital. They may have severe symptoms or get pneumonia. Or the flu infection may make an existing health problem worse.

How can you prevent it?

You can help prevent the flu by getting the flu vaccine every year. It's best to get the vaccine as soon as it's available. The National Advisory Committee on Immunization (NACI) recommends that everyone 6 months old and older get a flu vaccine. footnote 1

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Cause

The flu is caused by the influenza virus. Common classes of the virus are type A and type B. Each type includes several subtypes, or strains.

The influenza virus changes often. So having the flu caused by one strain doesn't give you full immunity to other strains.

The virus is spread from person to person through:

  • Direct contact, such as shaking hands.
  • Small droplets that form when a person sneezes or coughs.
  • Contact with objects, such as handkerchiefs, that have touched fluids from an infected person's nose or throat.

What Increases Your Risk

Anyone exposed to the influenza (flu) virus can get the flu. These flu viruses spread easily among people in groups. For example, people in nursing homes, hospitals, shelters, schools, and daycare can easily be exposed to the flu virus. Working, visiting, or living in any of these areas puts you at more risk of getting the flu.

The risk of having severe symptoms and complications is higher for:

  • Children younger than 5 years of age.
  • Adults age 65 and older.
  • Pregnant women.
  • People who have chronic obstructive pulmonary disease (COPD), other lung diseases, or heart failure.
  • People who have a medical condition (such as AIDS) or who are using a medicine that impairs the immune system.
  • People of any age who are residents of nursing homes and other chronic care facilities.
  • Indigenous peoples.

Prevention

You can help prevent influenza (flu) by getting the flu vaccine every year. It's best to get the vaccine as soon as it's available.

The National Advisory Committee on Immunization (NACI) recommends that everyone 6 months old and older get the vaccine, especially people at higher risk of problems from the flu. This includes:

  • Young children.
  • Adults age 65 and older.
  • Adults and children who have long-term health problems or a weak immune system.
  • Women who will be pregnant during flu season.

The vaccine is also important for health care workers and anyone who lives or works with someone at higher risk of problems from the flu.

You can also help avoid getting the flu by washing your hands often, keeping your hands away from your face, eating a balanced diet, exercising regularly, and not smoking.

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Symptoms

The symptoms of influenza (flu) appear suddenly. They often include:

  • Fever.
    • Fever is usually continuous, but it may come and go.
    • Fever may be lower in older adults than in children and younger adults.
  • Body aches and muscle pain (often severe).
  • A headache.
  • Pain when you move your eyes.
  • Fatigue, a general feeling of sickness, and loss of appetite.
  • A dry cough, a runny nose, and a dry or sore throat.

Symptoms are usually worse for the first few days. But it can take up to a few weeks to get completely better.

It usually takes 1 to 4 days to get flu symptoms after you've been around someone with the virus. Some people can have the flu virus without having any symptoms.

The flu usually doesn't cause symptoms in the stomach or intestines, such as vomiting and diarrhea.

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What Happens

Although most cases of influenza (flu) get better without causing other problems, complications sometimes develop.

Possible problems from the flu include:

  • Pneumonia, which is an infection of the lungs. It rarely causes death in young, healthy people. But it can often be life-threatening in older adults, people who have other diseases, and pregnant women.
  • Bronchiolitis. This is an inflammation of the small air passages. It affects infants and is the leading cause of serious lower respiratory illness.
  • Sinusitis. This means the mucous membranes that line the inside of the nose and facial sinuses become infected and inflamed.
  • Croup. Croup is a swelling or blockage in the windpipe. It causes hoarseness, a barking cough, a high-pitched sound when breathing in, and trouble breathing.
  • Chronic obstructive pulmonary disease (COPD) symptoms may get worse.
  • Inflammation of the heart muscle, the sac around the heart, or other muscles.

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When to Call a Doctor

Call 911 or other emergency services if:

  • You are having trouble breathing, or you feel very short of breath.
  • You have a severe headache or stiff neck and are confused or having trouble staying awake.

Call your doctor if:

  • You have an extremely high fever.
  • Your fever lasts longer than 3 days.
  • Your child is 3 months of age or younger and has a fever of 38°C (100.4°F) or higher.
  • Your cough lasts more than 7 to 10 days after other symptoms are gone.
  • You are coughing up yellow, green, rust-coloured, or bloody mucus.
  • You are finding it harder and harder to breathe.
  • Wheezing develops.
  • New pain develops or pain narrows to one area, such as an ear, the throat, the chest, or the sinuses.
  • Symptoms don't go away, even with home treatment.
  • Symptoms become more severe or frequent.

Watchful waiting

In most healthy people, the flu will go away in 5 to 7 days, although fatigue can last much longer. You may feel very sick, but home treatment is usually all that is needed. If it's flu season, you may just want to treat your symptoms at home. Watch closely for symptoms of a bacterial infection, such as nasal drainage that changes from clear to coloured after 5 to 7 days and symptoms that return or get worse.

Early treatment with an antiviral medicine may reduce the severity of influenza and may prevent serious flu-related complications. It's best to start these medicines right away. Babies, older adults, and people who have chronic health problems are more likely to have complications from the flu, and they may need to see a doctor for care beyond home treatment. But not all antiviral medicines work against all strains of the flu. Talk to your doctor if you think you may need an antiviral medicine.

Call your doctor if you think your symptoms are caused by something other than the flu.

Exams and Tests

Your doctor will ask you about your symptoms and will examine you. This usually gives the doctor enough information to know if you have influenza (flu), especially if many cases of a similar illness have occurred in the area and the local health unit reports a flu outbreak.

In some cases, the doctor may take a sample of fluid from your nose or throat. This may be done to confirm the flu. Or it can be done to find out what type of flu virus you have. Some tests give a result right away. Others take a few days.

A rapid flu test gives results quickly. Because rapid flu tests aren't completely accurate, your doctor may diagnose you with the flu even if the test doesn't confirm it.

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Treatment Overview

Most people can treat influenza (flu) symptoms at home. Home treatment includes resting, drinking plenty of fluids, and taking medicine to lower your fever. But some people need to go to the hospital for treatment. They may have severe symptoms or get pneumonia. Or the flu infection may make an existing health problem worse.

If you think you have the flu, your doctor may be able to give you an antiviral medicine that can make the symptoms milder and lessen the length of time you have the flu.

These antiviral medicines are often given to people who are very sick with the flu or to those who are likely to have problems caused by the flu. But they may also be used for a person who has been sick with the flu for less than 48 hours.

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Self-Care

If you have influenza, you can expect the illness to go away on its own in a few weeks. In the meantime, you can take steps to feel better:

  • Get extra rest.

    Bedrest can help you feel better. It will also help you avoid spreading the virus to others.

  • Drink plenty of fluids to replace those lost from fever.

    Water, soup, fruit juice, and hot tea with lemon are all good choices.

  • Sponge your body with lukewarm water to reduce an uncomfortable fever.

    Do not use cold water or ice. Lowering the fever will not make your symptoms go away faster, but it may make you more comfortable.

  • Take steps to control your cough.
    • Suck on cough drops or plain, hard candy.
    • Try an over-the-counter cough medicine. Read and follow all instructions on the label.
    • Raise your head at night with an extra pillow. This may help you rest if coughing keeps you awake.
  • Use petroleum jelly on sore skin.

    This can help if the skin around your nose and lips becomes sore from repeated rubbing with tissues. Using disposable tissues that contain lotion also may help.

  • Avoid smoking and breathing second-hand smoke.

    This is good advice anytime, but it is especially important when you have a respiratory infection like a cold or the flu.

  • Try an over-the-counter medicine to help relieve your symptoms.

    Acetaminophen (Tylenol), ibuprofen (Advil, Motrin), or naproxen (Aleve) may help relieve fever, headache, and muscle aches. Be safe with medicines. Read and follow all instructions on the label. No one younger than 18 should take aspirin. It has been linked to Reye syndrome, a serious illness.

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References

Citations

  1. Young K, et al. (2020). Summary of the NACI seasonal influenza vaccine statement for 2020–2021. Canada Communicable Disease Report, 46(5): 132–137. DOI: 10.14745/ccdr.v46i05a06. Accessed July 27, 2020

Credits

Current as of: June 12, 2023

Author: Healthwise Staff
Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.

This information does not replace the advice of a doctor. Healthwise, Incorporated, disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. Learn how we develop our content.

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