In all of human history, there had been no other disease like smallpox. It was vicious: It killed at least a third of those infected, and scarred or blinded most of the survivors. It was fast-moving, traveling everywhere that men had gone, carried by armies and traders and missionaries and slaves. It was relentless, roaring up into periodic epidemics, but never fully disappearing in between.
And it had always been there. The written records of its assaults on humanity went back more than three thousand years. Over the centuries, smallpox had devastated cities, changed the course of battles, and stalled the growth of empires. It had shuffled the leadership of dynasties like a pack of playing cards, decimating the royal houses of England, Holland, France, and Austria, and killing rulers in India, China, and Japan.
— From Beating Back the Devil