Why doesn't a whole town get sick at once?

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What else makes you wonder?

Why does super-catchy measles need almost everyone immune?

The easier a germ hops, the more paths it has. A germ that can leap to lots of neighbours needs a firewall of walls so thick that almost every path is closed.

Could the same firebreak idea stop a rumour — or a wildfire?

Anything that spreads by hopping neighbour to neighbour has a tipping point. Cut enough gaps in the chain and the wildfire runs out of trees, the rumour runs out of ears.

If you're too little for a vaccine, can the town still shield you?

You're a grey-blue person the germ never reaches — because it dead-ends on all the walls around you first. Who does that protect the most?

After you watchWhy doesn't a whole town get sick at once?

The short answer

A whole town often doesn't get sick because once enough people are immune, the germ runs out of people to hop to and fizzles out — protecting even people who aren't immune. You don't need everyone to be immune, just enough.

Try this next

What if the germ were twice as catchy? Start with fewer people immune, then step the outbreak — does the same wall of firebreaks still stop it, or does it break through?
What if you clustered all the immune people together? Tap to put the green 'walls' all in one corner, then start the germ on the open side — does the town's same total immunity still protect everyone?

Now you — bend it

What if What if you spread the immune people out evenly instead of clumping them in one corner?Even firebreaks block paths all over town, so the germ hits a dead end no matter where it starts — clumps leave one open pocket wide open.
What if What if you make the germ much catchier before adding any immune people?A catchier germ hops to more neighbors, so it needs more firebreaks to close every path — you may have to push the immune slider much higher to find the tipping point.
What if What if you start the outbreak right next to the immune wall instead of far away?The germ still needs an open path; if the wall surrounds it, it dead-ends fast even though it started close by.

Can you prove it?You don't need everyone immune to stop the germ — just enough to break its paths. — Set the immune slider below 100% and keep stepping it up. Find the point where the outbreak suddenly fizzles and can't reach the far side — that's the tipping point, and it lands well under everyone.

Design your own test:Before you start the germ, predict: will the same number of immune people protect the whole town better when they're spread out or clumped together?

Explain it to a 6-year-old: If enough friends hold hands to block the tag-player's path, they can't tag anyone — even friends who aren't holding hands.

The whole story

How it works

A germ spreads by hopping from a sick person to the people right next to them, building a chain. Immune people can't catch it and can't pass it on, so when the germ hops into one, that path hits a dead end. When enough people are immune and spread through the town, they break so many paths that the germ can't find a way across — it dies out before it reaches everyone. Scientists call this point the tipping point, or epidemic threshold.

What people get wrong

Many people think only those who aren't immune can get sick, and that you'd need everyone to be immune to stop a germ. Actually, once you pass a tipping point — usually well below 100% — the germ runs out of open paths and stops, so even people who aren't immune are protected because the germ never reaches them.

The catch

The magic number isn't fixed. A stickier, more contagious germ needs more people immune to stop it — a very catchy one like measles needs about 19 out of 20, while a milder germ needs far fewer. And it's not just how many people are immune, it's where they are: if all the immune people are clumped together, the germ can still tear through an unprotected pocket on the other side of town.

Questions kids ask

If I'm not immune, am I still protected?

Yes, you can be. Once enough other people around you are immune, the germ can't find a path to reach you, so it often stops before it ever gets to you. That's why "enough" matters more than "everyone."

How does a person become immune without getting sick first?

The usual way is a vaccine, which teaches your body how to beat a germ in advance. Having had the germ before can also make you immune. Either way, your body already knows how to win, so it can't catch it or pass it on.

Why does measles need so many people immune?

Because measles is extremely catchy — one sick person can spread it to many others. The easier a germ hops, the more immune people (firebreaks) you need to block all its paths, which is why measles needs about 19 out of 20.

Can a germ still spread if most people are immune?

Sometimes, if the immune people are all clumped in one place. A group of people who aren't immune, all near each other, gives the germ a pocket of open paths where it can still spread, even though the town overall has plenty of immune people.

Talk about it

Ask them: if 'enough' people being immune can stop a germ for everyone, why does it still matter that some people can't get a vaccine (like newborns)?
Ask: where else have you seen one blocked path stop a whole chain — dominoes, a traffic jam, a game of tag?

For grown-ups

This is the epidemic threshold, commonly called herd immunity. The fraction of a population that must be immune to halt sustained spread is roughly 1 − 1/R₀, where R₀ is the basic reproduction number — the average number of people one infected person would infect in a fully susceptible population. A higher R₀ demands higher coverage: measles (R₀ ≈ 12–18) requires about 92–95%. Mechanistically it behaves like percolation — above the threshold the connected chains of transmission collapse and outbreaks fade out. Immunity can come from vaccination or prior infection, and clustering of susceptible people lowers the protection a population actually gets, even at the same overall coverage.