Definition
Herd immunity is the state in which enough people in a group are immune to a disease that it becomes hard for the disease to keep spreading. Chains of infection keep breaking because they can't find someone to pass it to, so even people who aren't immune get exposed less. It only works for diseases where the chain of spread still runs from person to person, even if it passes through something in between, like mosquitoes or water.
Why does my shot protect people who didn't get one?
Some diseases, like measles, can't be prevented in newborns because they're too young for the vaccine. Still, if lots of people around a baby are immune, the baby has far fewer chances to run into the disease at all.
Picture a row of dominoes set up in a line. You tip the first one over. If the dominoes are packed close together, the fall races all the way to the end. But if a few pieces along the way are glued to the table, the fall stops right there and goes no further.
Diseases that pass between people work like a chain, too. One person passes it to the next, who passes it to the next. If someone immune is standing in the middle of the chain, it breaks right there.
So one person's immunity ends up covering the people standing behind them. The people being protected didn't have to do anything at all.
How often does the chain have to break?
What matters is how many people one sick person passes the disease to, on average. If that average is above 1, cases keep growing. If it's below 1, the chain fizzles out on its own.
As more people become immune, that average drops. Even when a sick person meets someone new, if that person is already immune, the chain stops there. So the point where the average falls below 1 is the threshold where an outbreak stops growing and starts to shrink. There is protection even before you reach that threshold. Every time a chain breaks, a little more protection builds up from the very beginning. The threshold is simply the point where the outbreak can no longer grow on its own.
The more contagious a disease is, the higher the share of immune people you need. If one person passes it to lots of others, more spots in the chain have to be blocked before it breaks. In domino terms, you'd need even more glued-down pieces packed in.
But this math only works under certain assumptions. People have to mix evenly, immune people have to be spread evenly around, and the immunity has to last. That immunity also has to stop people from passing the disease on, not just from getting sick, or the chain won't break. If the assumptions don't hold, the share you need changes too.
A bit more precisely
First, let's separate what the words refer to. Immunity is a state of one person's body. Herd immunity is a state of the whole group, where enough immune people make it hard for spread to continue. The first looks at your body. The second looks at the connections between people.
This idea doesn't work for every disease, either. The dividing line isn't how a disease spreads, but whether both ends of the chain are people. Malaria, which mosquitoes carry from one person's blood to another, and diseases that travel through water people have contaminated, still run from person to person in the end, so herd immunity works. By contrast, a disease like Japanese encephalitis cycles between animals and mosquitoes, so building immunity among people doesn't break the chain. And a disease like tetanus lives in soil and enters through wounds. It doesn't pass through people at all, so there's no chain to break, and herd immunity doesn't apply.
The share of people you need isn't one fixed number for each disease, either. How many people one person infects changes with place, season, and how closely people live and mix, and the share you need moves along with it.
The domino analogy holds up to a point. In both cases, a blocked spot in the middle stops the spread. But dominoes stand in a single line, while people are tangled in many branching connections, and who meets whom is different every time. That makes the real picture far more complicated than one line of dominoes.
Common misconceptions
Once you reach herd immunity, the disease disappears from the world.
The chain just becomes hard to keep going. The germ that causes the disease hasn't gone anywhere. If the share of immune people drops or the way people mix changes, it can spread again.
The share needed for herd immunity is one fixed number for each disease.
That share comes from how many people one person passes the disease to on average, and that number itself changes with place, season, and how people mix. It's calculated assuming people mix evenly, so it can differ from real life.
Herd immunity is a principle that applies to every disease.
It only works for diseases where the chain still runs from person to person. Whether a mosquito bites or people contaminate the water, it works as long as both ends of the chain are people. But a disease like tetanus lives in soil and enters through wounds without passing through people, so there's no chain to break. No matter how many people around you are immune, your path to infection doesn't shrink.
Where you meet it
A baby too young for the measles vaccine has far fewer chances to run into measles at all, thanks to the people around them.
In a classroom where most kids are already immune to a disease, if one child catches it, it often stops after a few cases instead of spreading to the whole class.
In one sentence
Herd immunity is a state of the whole group, where enough people are immune that chains of spread keep breaking, so even people who aren't immune are exposed less.