Though the origins of the Black Death are unclear, many scholars believe that the first recorded outbreaks of the plague took place in Mongolia in the late 1320s. According to one theory, long periods of intense rain forced rodent populations to flee their native habitats and migrate to new areas. Their movement brought them into more frequent contact with nomadic tribes on horseback who lived on the Steppes, a vast, level, treeless expanse of land in Central Asia. Here, the Great Mortality took its first human victims.

From Mongolia, the disease spread into China and other parts of Asia, turning human existence into a prolonged and unparalleled season of horror and torment. Like so many others around the world, millions of Asians already suffered from a series of natural calamities even before the plague arrived. Droughts and earthquakes...