When diseases spread
Most of the time, diseases spread at a steady, low level. But sometimes cases suddenly shoot up. Scientists use special words to describe how widely a disease is spreading.
| Term | Meaning | Example |
|---|---|---|
| Endemic | Always present at a steady level in an area | Malaria in parts of Africa |
| Outbreak | Sudden rise in a small area | Food poisoning at a single event |
| Epidemic | Rapid spread through a larger community or region | Ebola in West Africa, 2014–2016 |
| Pandemic | An epidemic that spreads around the world | COVID-19, declared a pandemic in 2020 |
A pandemic is about how far a disease spreads, not how deadly it is.
How diseases spread
Contagious diseases spread from person to person, or from animals to people.
| Route | Examples |
|---|---|
| Droplets and aerosols from coughs, sneezes or breathing | Flu, COVID-19, measles |
| Direct contact | Chickenpox |
| Contaminated surfaces | Cold viruses |
| Contaminated food or water | Cholera, norovirus |
| Insects and other animals (vectors) | Malaria (mosquitoes), Lyme disease (ticks) |
| Body fluids | HIV |
Many new diseases come from animals. These are called zoonotic diseases. Bird flu and Ebola are examples.
Measuring spread
Epidemiologists, scientists who study disease patterns, use a number called R (the reproduction number): the average number of people one sick person infects.
- If R is greater than 1, cases grow.
- If R is less than 1, cases shrink.
| Disease | Typical R₀ (in a population with no immunity) |
|---|---|
| Seasonal flu | About 1–2 |
| Original COVID-19 strain | About 2–3 |
| Measles | About 12–18 |
That’s why measles spreads so easily and why high vaccination rates are needed to stop it.
Why pandemics happen
- A new germ appears that people have no immunity to.
- It spreads easily between people.
- Air travel and big cities let it travel quickly around the world.
Fighting epidemics and pandemics
Public health experts work to protect whole communities.
| Tool | How it helps |
|---|---|
| Surveillance | Tracking cases to spot outbreaks early |
| Testing and contact tracing | Finding who’s sick and who they may have infected |
| Isolation and quarantine | Keeping sick or exposed people apart from others |
| Hand washing and masks | Reducing spread of germs |
| Vaccines | Preventing infection or serious illness |
| Treatments | Antivirals and antibiotics |
| Clean water and sanitation | Preventing waterborne diseases like cholera |
Flattening the curve
Slowing the spread spreads cases out over time, so hospitals aren’t overwhelmed all at once. This is called "flattening the curve."
Herd immunity
When enough people are immune, through vaccination or past infection, a disease can’t easily find new people to infect. This herd immunity protects people who can’t be vaccinated, such as newborns or people with weak immune systems.
Organizations that help
- The World Health Organization (WHO) coordinates global responses.
- The Public Health Agency of Canada protects Canadians’ health.
- Provincial and local health units track and respond to local outbreaks.
COVID-19
COVID-19 is caused by the SARS-CoV-2 coronavirus, first identified in late 2019. The WHO declared it a pandemic in March 2020. Scientists developed vaccines in under a year, much faster than ever before, building on decades of earlier research. The WHO reported about 7 million confirmed deaths, though the true toll is believed to be much higher. The WHO ended the global health emergency in May 2023.
Common mistakes
- Thinking a pandemic means a disease is very deadly. It means it’s very widespread.
- Thinking only humans spread disease. Animals and insects do too.
- Thinking you’re not contagious if you feel fine. Some people spread germs before or without feeling sick.
Try it yourself
- Simulate spread: one person "infects" two others each round. How many are infected after 5 rounds? Now try with one person infecting one other.
- Graph a "flattened" curve and a steep curve with the same total cases. Explain why the flat curve is better for hospitals.
- Research one disease that was eliminated from Canada by vaccination.
(Answer to 1: 1 + 2 + 4 + 8 + 16 + 32 = 63 people)
Quick recap
- Endemic, outbreak, epidemic and pandemic describe how widely a disease spreads.
- Contagious diseases spread through air, contact, food, water and vectors.
- R tells how many people one sick person infects.
- Public health measures and vaccines slow and stop spread.
- Herd immunity protects people who can’t be vaccinated.