Kangxi, the great 17th-century emperor of China, owed his throne to smallpox. His father had died of the disease at just 22 years old in 1661. Kangxi, only 7 years old, had been chosen over his older brother because Kangxi had already survived smallpox—an epidemic that was cutting a scythe through the ruling elite. Kangxi’s dynasty, the Qing, were Manchu, a steppe people who had conquered China in 1644. They had no concept of germ theory, but they could observe that the Chinese, living in far