The disease caused by the Variola virus that killed an estimated 300 million people in the 20th century alone, shaped the outcome of colonial conquest in the Americas and elsewhere through catastrophic differential mortality, and was eradicated by a global vaccination campaign in 1980 — the only human infectious disease to be deliberately eliminated from nature. Smallpox is simultaneously civilization's most lethal recurring catastrophe and its greatest public health victory.
In 1980, the World Health Organization declared that smallpox no longer existed in nature. For the first time in history, a disease that had killed hundreds of millions of people — that had changed the outcome of wars, toppled dynasties, and made European conquest of the Americas possible — had been deliberately hunted to extinction. No human effort has ever achieved anything quite like it. Understanding what smallpox did to civilization, and what civilization did to smallpox, is one of the clearest windows into both the power and the limits of organized human action.