Reviewed October 2022


Influenza, also known as the flu, is a contagious infection of the nose, throat and lungs. It is caused by influenza A and B viruses. Each year in Canada about five to 10 per cent of adults and 20 to 30 per cent of children are infected with influenza, usually in the late fall and winter.

Signs and Symptoms

Symptoms include sudden onset of fever, cough and muscle aches. Other common symptoms include headache, chills, loss of appetite, fatigue and sore throat. Nausea, vomiting and diarrhea may also occur, especially in children.


It can be difficult to distinguish influenza from other viral or bacterial causes of respiratory illness based on symptoms alone. There are laboratory tests that can be done to diagnose flu, although it is not usually necessary for most people. Testing can be done by swiping the inside of your nose with a swab to collect the virus.


Most people will recover within a week to ten days. Some people are at greater risk of complications which include pneumonia, ear infections, sinus infections, dehydration and worsening of chronic medical conditions, such as heart disease, asthma and diabetes. Each year in Canada, about 12,200 people are hospitalized and 3,500 die due to influenza and its complications.

Risk Factors

People at high risk of influenza-related complications or hospitalization include:

  • those with chronic health conditions, including diabetes, heart and lung disease
  • people who live in long-term care homes and other chronic-care facilities
  • all pregnant women
  • Indigenous peoples
  • children under five years of age
  • persons 65 years of age and older


Most healthy people who get the flu will not need medical treatment. Antiviral drugs are prescription drugs that may be used to lessen symptoms and prevent complications. Antiviral drugs are usually used early to treat hospitalized patients, people with severe flu illness, and people who are at higher risk for flu complications based on their age or other health conditions. For more information, see What You Should Know About Flu Antiviral Drugs (Centers for Disease Control and Prevention), or speak with your healthcare provider.


Getting the flu vaccine is the best way to prevent influenza. Flu vaccine prevents illness, doctor’s visits and hospitalizations. Each year, there is a new vaccine to protect against the flu virus strains that are expected in the coming influenza season. Even if the strains have not changed, getting vaccinated every year is needed to maximize protection as immunity wears off within a year.

Other steps you can take to prevent influenza infection include clean your hands frequently and avoid touching your eyes, nose and mouth with unclean hands; cover your cough and sneeze; stay away from people who are sick; and stay home when you are sick.


If you have the flu, the Ontario Ministry of Health and Long-Term Care recommends the following:

  • Stay home and get plenty of rest.
  • Drink lots of fluids.
  • Avoid caffeine.
  • Speak to your doctor, nurse practitioner or pharmacist about over-the-counter medications that can help you feel better (such as basic pain or fever relievers) but do not give acetylsalicylic acid (ASA or Aspirin®) to anyone under the age of 18 years old.
  • Treat muscle pain using a hot water bottle or heating pad — apply heat for short periods of time.
  • Take a warm bath.
  • Gargle with a glass of warm salt water or suck on hard candy or lozenges.
  • Use spray or saline drops for a stuffy nose.
  • Avoid alcohol and tobacco.

 Call your doctor or nurse practitioner if:

  • you don’t start to feel better after a few days
  • your symptoms get worse
  • you are at high risk of developing influenza-related complications and develop flu symptoms

You can also call Telehealth at 1-866-797-0000 to talk to a registered nurse 24 hours a day, 7 days a week. You do not need to provide your OHIP number and all information is confidential.


Influenza virus is mainly spread by droplets made when people with influenza cough, sneeze or talk. These droplets can then land in the mouths, noses or eyes of people who are nearby (within two metres). Less commonly, a person may also get influenza when they touch a surface or object that has influenza virus on it and then touch their own mouth, eyes or nose.

It can take from one to four days to develop influenza illness after being infected with the virus. People with influenza may infect others beginning from one day before symptoms start until about five days after becoming sick. Children and people with weak immune systems may spread virus in droplets for longer.

More Information

Call Toronto Public Health at 416-338-7600 (TTY at 416-392-0658) or speak to your healthcare provider.