The excellent Jason Crawford at the Roots of Progress has a long-form read on the history of smallpox eradication. It’s an important and insightful piece especially because Jason is interested not just in what happened but why it happened when and where it did and what the lessons are for today:

In 1720, inoculation had been a folk practice in many parts of the world for hundreds of years, but smallpox was still endemic almost everywhere. The disease had existed for at least 1,400 and probably over 3,000 years. Just over 250 years later—it was gone.

Why did it take so long, and how did it then happen so fast? Why wasn’t inoculation practiced more widely in China, India, or the Middle East, when it had been known there for centuries? Why, when it reached the West, did it spread faster and wider than ever before—enough to significantly reduce and ultimately eliminate the disease?

The same questions apply to many other technologies. China famously had the compass, gunpowder, and cast iron all before the West, but it was Europe that charted the oceans, blasted tunnels through mountains, and created the Industrial Revolution. In smallpox we see the same pattern. [Why?]