Why doesn't a whole town get sick at once?
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Herd Immunity
Watch immune neighbors break the germ’s paths, helping protect people who cannot be vaccinated.
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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?
More to explore
After you watchWhy doesn't a whole town get sick at once?
The short answer
Immune people can interrupt chains of infection. This can reduce the chance that a germ reaches someone who is not immune. Community protection depends on the infection, patterns of contact, and how well immunity works; it does not guarantee that nobody gets sick.
Try this next
- What changed when more people started immune? After the first run, move the immunity slider back below 60%, then raise it to a different level. Compare the numbers actually reached; do not assume every run ends the same way.
- Does one run prove a rule for every town? Name two things this model leaves out, such as people travelling or immunity being incomplete.
Now you — bend it
- What if immune people were grouped together instead of spread out?Sketch two towns with the same number of immune people. Mark possible paths without claiming that the toy has tested your drawing.
- What if people could travel across town?Add one long-distance connection to your sketch. Could it join places that the neighbor-only model kept apart?
Can you prove it?An immune neighbor can block a transmission path. — Use the practice grid to mark two immune neighbors. Run it and trace a path that stopped. Distinguish what this run showed from what it cannot prove about all towns.
Design your own test:Draw two contact networks with the same people and same immune group. Predict where the infection could travel before tracing the open paths.
Explain it to a 6-year-old: A germ needs a path from person to person. People with protection can help break some paths.
The whole story
How it works
In this simplified model, a germ can spread only to neighboring cells, and green immune cells block it completely. A fixed example with about 60% immune is run after the prediction; its measured count is reported without selecting a lucky outcome. Real immunity and contact networks are more complicated, and one model run cannot establish a real-world threshold.
What people get wrong
Immunity can help protect other people as well as the immune person, but there is no single percentage that works for every infection or every community. Neither vaccination nor prior infection guarantees complete protection against every germ.
The catch
Very contagious infections such as measles require very high community protection. Where susceptible people gather matters too: an outbreak can spread through a poorly protected pocket even when the wider community has high coverage.
Questions kids ask
If I'm not immune, am I still protected?
Community immunity can reduce your chance of exposure, but it does not guarantee protection. It matters especially for people who cannot receive some vaccines or do not develop a strong immune response.
How does a person become immune without getting sick first?
Vaccines train the immune system to recognize a germ without having to get the disease first. Protection varies by vaccine, infection, and person, and can change over time.
Why does measles need so many people immune?
Measles spreads very easily. Very high vaccination coverage — around 95% — helps provide community protection, although local gaps can still allow outbreaks.
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
- Which people were protected directly, and which people may have benefited because a path was blocked?
- Why might a real town behave differently from a grid of neighbors?
For grown-ups
The toy uses a four-neighbor grid with perfect immunity, fixed immunity placement, and a seeded stochastic transmission process. The seed is chosen in advance and is not rerolled to obtain a particular outcome. The animated run and calm snapshots use the same rules and seed. It illustrates blocked transmission paths, not a clinical prediction. In a homogeneous model with perfect immunity, the familiar threshold is approximately 1 − 1/R₀; real contact patterns, vaccine effectiveness, and waning immunity complicate that approximation.