What you're seeing
A crowd of people moving around a room. One person starts sick (red). When a sick person gets close enough to a healthy person, the disease may jump between them.
How to read the graph
Gray dots are healthy, red dots are actively infected, green dots have recovered and can't be re-infected. The mini chart in the corner tracks the epidemic curve over time.
The key lesson
One sick person can infect an entire population through chain reactions. Small changes in how contagious a disease is — or how close people stand — dramatically change whether an outbreak fizzles or explodes.