Definition
A state in which no new cases of a disease occur in nature anywhere on Earth. When a disease disappears only in a set region, that is called elimination, and it stays that way only if prevention continues there. Eradication means you can stop prevention altogether, so the whole planet has to be checked before it is declared.
Why don't young people have a smallpox vaccine scar?
Some older adults have a round scar on the upper arm. It is the mark of the smallpox vaccination they received. But people born in recent decades don't have one.
As cases of the disease dwindled, vaccinating everyone was stopped first, and then came the announcement that the disease had disappeared from the world. Among diseases that infect people, smallpox is still the only one to get this far. If you widen the view to animals other than humans, there is one more, rinderpest, which struck cattle. So the total so far is two.
What gives this announcement its weight is that the work of prevention can stop. It doesn't mean no patients have shown up for a while. It means that even if you let go, it does not come back. That is why the word is used so rarely and so late.
Here is the yard version. You stop pulling weeds altogether, and the next year nothing comes up. That is a different thing from having weeded well this year.
Gone from one country versus gone from the Earth
Some countries go years without a single case of a disease. Yet they keep their vaccination schedules and testing exactly as they are. Why?
Because seeds are still left beyond the fence. If the disease remains in other regions, a single flight can bring it back, and if prevention stops, it settles in again before long. Elimination means the disease no longer arises within a set region. Eradication means that is true across the entire planet.
That is why no country can reach eradication alone. If it remains in a neighboring country, only your own side of the yard is clean, and that is elimination at best. Until the very last place is cleared, no country can let go.
The two words cover very different scales, so when you hear on the news that something is "gone," check first which one is meant. Whether it is about your region or the whole planet completely changes what has to happen next.
A bit more precisely
Eradication is not the name of a method. Vaccination, surveillance that finds patients, and isolation that keeps patients apart are all roads toward it, and eradication is the result, the state you reach at the end of the road. How you did it and where you ended up are answers at different levels.
You also can't set this goal for just any disease. There are conditions. The germ must not hide anywhere besides humans, there must be a way to tell who is infected, and there must be a means to cut off transmission. If even one of the three fails, a last pocket remains somewhere.
Polio has long been pursued with this goal too. But some regions still have it, so eradication has not been declared, and the world keeps up vaccination and surveillance.
The weed analogy works up to this point. A single remaining plant can refill the yard the next year, in both cases. The difference is that weeds are visible, while a disease can linger unnoticed in people who have only a mild illness and move on, so confirming that it is gone is a big job in itself.
Common misconceptions
If a country has had no cases for several years, the disease has been eradicated.
When new cases stop occurring within one region, that is elimination. If the disease remains outside, it can come back in, so prevention has to continue. Eradication is used only when that is true across the whole planet.
Smallpox is the only disease humans have ever wiped off the Earth.
Among diseases that infect people, smallpox is the only one. If you widen it to animals other than humans, rinderpest, which struck cattle, has also been declared eradicated, so the total is two.
Eradicated means not a single sample of the germ is left, even in a laboratory.
Eradication refers to a state in which no new infections occur in nature. Whether samples kept for research have also disappeared is discussed as a separate matter.
Where you meet it
Smallpox vaccination is no longer given to everyone because the disease was eradicated, so there is no need to start that prevention again.
A country keeps its measles vaccination schedule even after years without a case, because that state is elimination, not eradication.
In one sentence
A state in which no new cases of a disease occur in nature anywhere on Earth, so prevention can stop; disappearing only from one region is called elimination, a separate thing.