How Infections Spread
Learning Objectives
Students will be able to:
- Identify the three main routes of respiratory infection transmission
- Differentiate between droplets and aerosols
- Explain how respiratory pathogens leave and enter the body
- Recognize that many infections spread through the air we breathe
The Big Question
"Think about the last time you had a cold. How did the germs get from someone else into your body?"
Three Routes of Transmission
1. Contact Transmission
How it works: Germs on surfaces (doorknobs, desks) transfer to your hand, then to your face.
Prevention: Wash hands, don't touch face, clean surfaces
2. Droplet Transmission
How it works: Large respiratory particles (>10 μm) from coughs/sneezes travel 3-6 feet before falling.
Prevention: Distance, cover coughs, barriers
3. Aerosol (Airborne) Transmission
How it works: Tiny particles (<5 μm) float in the air for minutes to hours, travel across rooms.
Prevention: Ventilation, filtration, masks, fresh air
Droplets vs. Aerosols
| Feature | Droplets | Aerosols |
|---|---|---|
| Size | >10 micrometers | <5 micrometers |
| Behavior | Fall within seconds | Float for minutes to hours |
| Travel distance | 3-6 feet (1-2 meters) | Across entire rooms |
| Protection | Distance works | Need ventilation + filtration |
| Examples | Visible spray from sneeze | Invisible cloud from breathing |
What Comes Out When We Breathe
Even normal breathing releases thousands of particles:
| Activity | Particles per Minute | Mostly Aerosols? |
|---|---|---|
| Breathing quietly | 200-1,000 | Yes (>80%) |
| Talking | 1,000-5,000 | Yes (>80%) |
| Singing | 5,000-50,000 | Yes (>80%) |
| Coughing | 20,000-200,000 | Mixed (50-80%) |
| Sneezing | 200,000-1,000,000 | Mixed (50-80%) |
Surprise: You don't need to cough or sneeze to spread infection—just breathing and talking releases infectious particles!
Why Indoor Air Matters
Outdoors
- Wind dilutes particles quickly
- Infinite volume to spread into
- UV light inactivates some germs
- Low risk
Indoors
- Particles accumulate in enclosed space
- Without ventilation, concentration rises
- Longer time = higher exposure
- Higher risk
Common Misconceptions
Misconception
"You can only get sick if someone coughs on you"
Reality
Normal breathing releases infectious particles; sharing indoor air is enough
Misconception
"6 feet apart is always safe"
Reality
Aerosols travel across entire rooms; shared air matters more than distance
Key Takeaway
Many respiratory infections spread primarily through aerosols—tiny particles that float in the air. This means you don't have to be close to someone or be coughed on to get infected. Sharing indoor air with an infected person is enough. This is why ventilation, filtration, and masks are so important.