What is the flu?
Influenza, or the flu, is a contagious illness of the nose, throat and lungs caused by influenza viruses. It’s much more serious than a common cold. Each year in Canada, flu sends thousands of people to hospital.
Flu vs. a cold
| Symptom | Flu | Cold |
|---|---|---|
| Start | Sudden | Gradual |
| Fever | Common, often high | Rare |
| Body aches | Common, often severe | Mild |
| Tiredness | Can last weeks | Mild |
| Stuffy nose, sneezing | Sometimes | Common |
| Complications | Can lead to pneumonia | Rare |
How the flu spreads
The flu spreads mostly through droplets and tiny particles when sick people cough, sneeze or talk. It can also spread when people touch surfaces with the virus and then touch their eyes, nose or mouth. People can spread the flu from about a day before they feel sick to about a week after.
Seasonal flu
In Canada, seasonal flu usually peaks between November and April. Cold, dry air helps the virus survive, and people spend more time indoors together.
Types of flu viruses
| Type | Infects | Notes |
|---|---|---|
| Influenza A | Humans, birds, pigs and other animals | Causes most epidemics and all known pandemics |
| Influenza B | Mostly humans | Causes seasonal outbreaks |
| Influenza C | Humans | Usually mild |
Influenza A viruses are named for two surface proteins: hemagglutinin (H) and neuraminidase (N). That’s where names like H1N1 and H3N2 come from. Each version is called a strain.
Why we need a new vaccine every year
Flu viruses change constantly.
- Antigenic drift: small mutations happen all the time. Over a year or two, the virus changes enough that your old immunity doesn’t fully protect you.
- Antigenic shift: occasionally, two different flu viruses (such as a bird flu and a human flu) infect the same animal, often a pig, and swap genes. This can create a brand-new strain that almost no one is immune to, causing a pandemic.
Every year, the World Health Organization studies which strains are spreading and recommends which to include in the next season’s flu vaccine. It takes about six months to make the vaccine.
Flu pandemics
| Year | Name | Strain | Estimated deaths worldwide |
|---|---|---|---|
| 1918–1920 | "Spanish flu" | H1N1 | About 50 million |
| 1957–1958 | Asian flu | H2N2 | About 1–2 million |
| 1968–1969 | Hong Kong flu | H3N2 | About 1–4 million |
| 2009–2010 | Swine flu | H1N1 | About 150 000–575 000 |
Scientists closely watch bird flu strains like H5N1, which has spread widely among wild birds and some mammals in recent years, in case they begin spreading easily between people.
Who is at higher risk?
- Babies and young children
- Adults over 65
- Pregnant people
- People with chronic conditions like asthma, heart disease or diabetes
Prevention
| Action | Why it helps |
|---|---|
| Get the yearly flu vaccine | Reduces the chance of getting sick and of severe illness |
| Wash hands often | Removes viruses |
| Cough or sneeze into your elbow | Keeps droplets from spreading |
| Stay home when sick | Protects others |
| Keep surfaces clean | Removes viruses |
Treatment
- Rest and drink plenty of fluids.
- Fever and pain relievers can ease symptoms (follow the label).
- Antiviral medicines, prescribed by a doctor, can shorten the illness if started within about two days.
- Antibiotics don’t work on the flu, because it’s a virus.
Get medical help right away for trouble breathing, chest pain, confusion, or a fever that won’t go down.
Common mistakes
- Calling a bad cold "the flu." The flu is usually much worse.
- Thinking the flu vaccine gives you the flu. It can’t; the virus in the shot is inactivated or only a piece of it.
- Thinking last year’s flu shot still protects you.
Try it yourself
- Make a poster comparing flu and cold symptoms.
- Explain antigenic drift using an analogy, like a criminal slowly changing disguises.
- Look up this season’s flu activity in Canada on the government’s FluWatch+ reports.
Quick recap
- Influenza is a contagious respiratory virus, more serious than a cold.
- Seasonal flu peaks in winter.
- Influenza A strains are named for H and N proteins.
- Drift requires a new vaccine each year; shift can cause pandemics.
- Vaccines, hand washing and staying home when sick help prevent the flu.