1Two things to know first
Germs hop, and some people are walls
You need two small ideas. Watch each one happen:
Germs hop
A germ spreads by hopping from a sick person to the people right next to them.
Immune = a wall
An immune person can't catch it and can't pass it on — the germ hits them and stops.
2It all depends on the paths
Open path, or dead end
Open path 🦠
If the germ keeps finding people to hop to, the chain runs on and the whole town gets sick.
Hits a wall 🧱
If the germ hops into an immune person, that path is a dead end. Enough walls and it can't get across.
3Play with an outbreak
Start a germ and see it spread
Press start with nobody immune and watch it sweep the town. Then tap people to make them immune (they turn green) and start again — see what the walls do.
Tap any person to toggle them immune. Press start to send a germ through.
4Now find the magic number
How many need to be immune?
Slide how many people start out immune, then send a germ. Somewhere there's a tipping point where the town stops getting sick — but where?
Guess before you slide
To stop a germ from reaching the whole town, how many people need to be immune?
You can also tap people to make them immune. Then start the germ and watch what happens.
5But wait…
Two honest catches
A super-catchy germ like measles needs about 19 out of 20 people immune to stop it. A milder germ needs far fewer. The magic number depends on how easily the germ hops.
It's also where. If all the immune people are clumped in one spot, the germ can still tear through an unprotected pocket on the other side of town.
A germ spreads by hopping person to person. Once enough people are immune, it runs out of people to hop to and stops — protecting even those who aren't immune. That's why enough beats everyone.
Psst, grown-ups: this is the epidemic threshold (herd immunity). The share of a population that must be immune to halt sustained spread is roughly 1 − 1/R₀, so a higher basic reproduction number demands higher coverage (measles, R₀ ≈ 12–18 → ~92–95%). It's a percolation effect — above the threshold, connected transmission paths collapse and outbreaks die out. Immunity can come from vaccination or prior infection, and clustering of susceptible people lowers the effective protection a population gets.