Flu facts

What is the flu?

The flu, also known as influenza, is a common respiratory (nose, throat, and lung) infection. It can cause fever, muscle aches, extreme tiredness, sore throat, headaches, and a runny or stuffy nose (see "Flu symptoms" to learn more). About 10% to 20% of Canadians get the flu each year.

What causes the flu, and how does it spread?

The flu is caused by viruses. Each year there are different viruses that cause the flu. There are 3 main types of flu viruses: influenza A, influenza B, and influenza C. Influenza A and B are the most common. They cause flu during the winter months, also called "seasonal flu" (in Canada, flu season usually lasts from early winter to early spring). Influenza C viruses cause a mild illness and do not cause seasonal flu.

You may have seen other flu virus names such as H1N1 or H3N2 in the news. This is because influenza A viruses can be broken down into subtypes based on 2 proteins on their surfaces: H (which stands for hemagglutinin) and N (which stands for neuraminidase). This is where virus names such as H1N1 or H3N2 come from. Sometimes a new type of influenza A virus comes out that most people are not immune to. This virus can then spread quickly among people around the world. This is known as a flu pandemic.

You can catch the flu if a flu virus gets into your eyes, nose, or mouth. This may happen if an infected person coughs or sneezes near you, or if you touch a surface (such as a doorknob or elevator button) that has been touched by an infected person and then touch your eyes, nose, or mouth.

Flu symptoms usually appear 2 to 3 days after you are exposed to the virus, although they may appear anywhere from 1 to 7 days after you are exposed. The flu is highly contagious and can spread very quickly through a group of people.

How is the flu diagnosed?

Your doctor can usually diagnose the flu based on your symptoms alone. Your doctor may also do a physical exam, and may recommend other tests such as sputum cultures or chest X-rays to check for flu complications such as pneumonia.

Sometimes it can be hard to tell if you have the flu or a cold. Use the "Is it a cold or the flu?" tool to help you tell the difference.