Influenza, also known as the flu, is a virus that spreads easily and can infect 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.

How Influenza Spreads

Influenza is spread by droplets made when people with the virus cough, sneeze or talk. These droplets can then land in the mouths, noses or eyes of people who are nearby (within two metres). It has also been found that some respiratory viruses, including influenza, can spread through very small droplets called aerosols which can hang in the air for longer periods of time. Less commonly, a person may also get influenza when they touch a surface or object that has the virus on it and then touch their own mouth, eyes, or nose.

It can take from one to four days to develop the 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 the virus for a longer period of time.

Signs and Symptoms

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


It can be difficult to tell influenza from other viral or bacterial causes of respiratory illness as the symptoms are similar. If a health care provider needs to know if you are sick with influenza, they can test for it by swiping the inside of your nose with a swab to collect the virus and send to a lab.

Get tested and treated for COVID-19, if eligible.


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

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

  • People with chronic health conditions, including diabetes, heart, and lung disease
  • People who live in long-term care homes and other chronic-care facilities
  • Pregnant people
  • Indigenous people
  • Children under five years of age
  • People 65 years of age and older


Most healthy people who get influenza 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 influenza illness, and people who are at higher risk for 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 a health care provider.


Getting the flu vaccine each year 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 influenza virus strains that are expected in the coming respiratory virus season. Even if the strains have not changed, getting vaccinated every year is needed for the best protection as immunity wears off within a year.

Other steps you can take to prevent influenza infection include clean your hands frequently with soap and water or alcohol-based hand-sanitizer; avoid touching your eyes, nose and mouth with unclean hands; wear a well-fitting mask in crowded indoor settings, especially if you are at higher risk for severe illness; clean frequently touched surfaces such as doorknobs and mobile devices; cover your cough and sneeze with a tissue or your shirt, not your hands; stay away from people who are sick; and stay home when you are sick.


If you have respiratory virus symptoms/influenza, the Ontario Ministry of Health recommends the following:

  • Stay home and get plenty of rest
  • Drink lots of fluids
  • Speak to a health care provider 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
  • For muscle pain, use 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, except for young children
  • Use spray or saline drops for a stuffy nose
  • Avoid caffeine, alcohol, and tobacco

Contacting a Health Care Provider

If you have questions about your illness or you don’t start to feel better after a few days or your symptoms get worse, call a health care provider, or 811 (TTY: 1-866-797-0007) to talk to a registered nurse 24 hours a day, 7 days a week. Calls to 811 do not need you to provide your OHIP number and all information is free, secure, and confidential.

Caring for Children with Respiratory Symptoms

The Ontario College of Family Physicians have created a fact sheet with tips for parents/guardians on caring for children with respiratory symptoms, including how to help support your sick child at home and when to call a health care provider for a respiratory illness.

Speak to a healthcare provider or call 811 (Health Connect Ontario) if you have questions about your child’s health.

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More Information

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