Every outbreak starts with one sick person. The key number is R: how many people, on average, each sick person infects.

Try it: pick R = 3 and press Next generation four times. Then try R = 0.8 and R = 1.

• R above 1: each generation is bigger than the last — the outbreak grows exponentially.
• R below 1: chains shrink and die out.
• R = 1 is the tipping point.