Can humanity erase a disease from the Earth?

That time, that place

Smallpox killed more than three hundred million people in the twentieth century alone. The vaccine was by then a hundred and eighty years old.

And still it persisted, because vaccinating everyone on Earth was impossible. Poor countries had neither refrigeration nor health workers in the numbers required.

Why this question

So the strategy changed.

Instead of vaccinating everyone, vaccinate densely around each case: the contacts, and the contacts of contacts. Cut the chain the disease travels along.

This works with far less vaccine and far fewer workers. In exchange, finding cases fast becomes everything.

What was found

Several conditions made the plan possible.

Smallpox infects only humans; it does not hide in animals and return. Its symptoms are unmistakable, so cases are found quickly. And it is not transmitted by people without symptoms. Few diseases meet all three.

One more thing. In the middle of the Cold War, the United States and the Soviet Union worked together on this. The Soviets proposed it; the Americans supplied much of the funding and organization.

The last naturally occurring case appeared in Somalia in 1977. In 1980 the World Health Organization declared smallpox eradicated.

It remains the only human infectious disease humanity has removed from the Earth.