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The Facts

The flu is a respiratory (i.e., nose, throat, and lung) infection that can be caused by  influenza viruses. Many people use the word "flu" when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.

In North America, flu almost always strikes between October and April. Most people who get the flu will recover within 7 to 10 days, but some people are at risk of developing complications such as pneumonia. On average, about 3,500 people in Canada die each year from complications of influenza, and about 12,000 people with the flu are hospitalized. Most of these people have other medical conditions, are seniors, or are very young children.


Influenza is contagious, which means it can be spread easily from person to person. Viruses that cause influenza spread from person to person mainly by droplets of respiratory fluids that are sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.

The flu can also be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches his or her nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.

There are three types of influenza viruses: A, B, and C.  Influenza A and B are associated with seasonal influenza and most outbreaks of the flu. Influenza C is rarer and does not usually cause outbreaks. Type A influenza poses the most serious problems for humans. Strains of this type have also been found in birds, pigs, and other animals. Viruses that affect two different species sometimes combine and mix-and-match genetic information to create a new strain that nobody is immune to and for which no vaccine has been prepared.

The flu takes 1 to 4 days to incubate in humans, but infected people become contagious before symptoms appear, often just the day after the virus enters the body. Adults remain infectious (i.e., they can spread the virus to others) for about 5 to 7 days, and young children may remain infectious for a few days longer.

Symptoms and Complications

Initial flu symptoms include headaches, chills, and a cough. Symptoms such as fever, loss of appetite, and muscle aches are soon to follow. Other symptoms such as nausea, vomiting, and diarrhea are rare in adults but more common in children.

Since many people think they have the flu when it's actually a bad cold, here's a quick guide to help you tell the difference:






Usually present, high (38°C to 40°C or 100°F to 104°F); lasts 3 to 4 days



Very common

Aches and pains


Common and often severe

Fatigue and weakness


Can last 2 to 3 weeks

Extreme exhaustion


Very common at the start

Stuffy nose






Sore throat



Chest discomfort and cough

Mild to moderate, hacking cough


While most people recover in 7 to 10 days, severe illness can occur. The main complications are secondary bacterial infections of the sinuses or lungs (pneumonia). Symptoms include fever, chills, body aches, and cough. Children are prone to ear infections like otitis media.

People in nursing homes are at a higher risk of complications from flu because they may have weak immune systems and often have other medical problems. People with asthma, chronic obstructive pulmonary disease, or congestive heart failure are also at a higher risk of developing bacterial infections like pneumonia. In fact, influenza and pneumonia are ranked among the top 10 leading causes of death in Canada.

Making the Diagnosis

Since the symptoms of an influenza infection can vary from the common cold, a diagnosis can be made fairly quickly. However, many symptoms of the flu are similar to respiratory infections. If you think you have the flu, it is important to contact your health care provider first to make an appointment. Your doctor will be able to tell that you have the flu if you have at least some or most of these symptoms:

  • aches and pains everywhere, especially in the back and legs
  • bad headache
  • burning sensation in the chest
  • dry cough at first, then bringing up sputum
  • high fever
  • nauseous feeling and possible vomiting
  • sore throat
  • runny or stuffy nose
  • extreme tiredness

If there is any doubt, your doctor can make sure that it's the flu by taking a throat swab and testing it for the virus.

Treatment and Prevention

The normal treatment for flu is rest and plenty of liquids. Treatment also includes ways to prevent spreading the flu virus, such as proper handwashing, keeping common surfaces clean, and coughing or sneezing into your arm or sleeve.

Medications for specific symptoms can help. Cough suppressants can be used for cough. Acetylsalicylic acid* (ASA), ibuprofen, or acetaminophen can be used to treat symptoms of the flu, such as aches and fever. Children and teenagers with flu shouldn't take ASA or other salicylates (medications related to ASA, such as salsalate or magnesium salicylate). The combination of influenza and ASA is linked to Reye's syndrome, a rare but serious condition affecting the brain and liver. Many over-the-counter cold medications contain ASA or other salicylates. Ask your doctor or pharmacist about this.

Antibiotics are not effective against viral infections like flu and the cold, but they may be prescribed for complications, such as pneumonia.

Antiviral medications are sometimes used to treat the flu. These medications can help shorten the duration of the flu and reduce symptoms if they are taken within 2 days of the start of symptoms. Antiviral medications are also recommended to prevent flu infection for some people. Antivirals can be used to prevent flu in children and adults after they come into close contact with a person who has the flu, such as flu-infected people who live in the same household. Generally, this is not recommended for most people; however, antivirals may be recommended for people at risk for flu complications. In these situations, antiviral medications should be started as soon as possible after becoming exposed to the person with the flu. Your doctor can decide whether you should start antiviral medications. 

Zanamivir and oseltamivir are antiviral medications that can be used to treat and prevent influenza A. They prevent newly formed viruses from escaping the infected cells that produced them. This limits further spread of the virus in the body. Zanamivir is an inhaled spray, whereas oseltamivir is a pill. Taken within 24 to 48 hours after the onset of illness, these medications reduce the duration of symptoms by up to 1 day.

Flu antibodies can prevent flu. The only ways to generate antibodies are to be infected or to get vaccinated. Because the flu viruses can change from year to year, vaccination needs to be repeated every year.

Each spring, a worldwide network of physicians and testing labs decide which flu strains are likely to cause trouble and design that year's vaccine accordingly. The vaccine is designed to work against the type B strain and the 2 type A strains that are expected to predominate in the coming flu season.

An annual flu shot is the most effective way to prevent the flu and its complications.  However, there is still a chance that you can get the flu – although your symptoms may be milder. The flu vaccine is recommended for anyone aged 6 months and older.

Certain people are at an increased risk of complications from the flu and should receive the vaccine. You are at high risk and should be vaccinated if you:

  • are aged 65 years or older
  • live in a nursing home or long-term care facility
  • have a lung disease (e.g., asthma, chronic obstructive pulmonary disease [COPD])
  • have a heart condition (e.g., angina, congestive heart failure)
  • have diabetes or another metabolic disease
  • have a kidney problem
  • have a blood disorder (e.g., anemia)
  • have a neurologic or neurodevelopment condition
  • have cancer or a weakened immune system (e.g., are taking steroid medications, or have HIV)
  • are pregnant
  • are aged 6 months to 59 months
  • are aged 6 months to 18 years and are taking long-term ASA therapy
  • have been diagnosed as suffering from morbid obesity (BMI of 40 or higher)

People who should not receive a flu shot include children less than 6 months of age and people with severe allergies to the flu vaccine or any ingredient in the vaccine.

You can reduce your risk of getting the flu by practicing regular hand-washing using soap and warm water or an alcohol-based hand sanitizer. Also, cough or sneeze into a tissue or into your sleeve. Dispose of the tissue right away. If you have flu symptoms, stay home from work or school and avoid contact with people who are at a high risk of flu complications (e.g., seniors, nursing home residents).

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