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