A child on a hill path looks down at a small town of houses where the people are calm grey-blue figures, and one person in the middle has just caught a germ and turned coral-red.

Why doesn't the WHOLE town get sick?

One person catches a germ. You'd think it would race through everyone… but often it fizzles out fast. What stops it?

First clue: a germ spreads by hopping from a sick person to whoever stands right next to them. Tap to send one down the street.

tap to send the germ hopping

Second clue: some people are immune (green). In this model, immune people cannot catch or pass on the germ. Each one blocks a path. Real protection is not always complete.

A row of people: a coral-red sick person on the left, the germ hopping in dotted coral arcs and turning people coral one by one, until it reaches a single green immune person where the last hop bumps off and fades, leaving the grey-blue people beyond untouched.

Here's a whole town. Press start with nobody immune — see how far the germ travels. Then tap a couple of people green and see it swerve.

can catch it sick immune (tap to add) all better

Arrow-keys and Enter add a green wall too.

A germ can travel around isolated immune neighbors.

A top-down town with many people scattered through the streets, a good number of them now green immune figures mixed evenly among the grey-blue ones, with a single coral-red sick person just starting at one edge.

So sprinkle in LOTS more immune people (green). Is that finally enough to trap it? 🤔

You tested just two immune neighbors. Now try a new arrangement.

In a new model town with about 60% immune, how many of the remaining people will this outbreak reach?