Modern scientists believe that the Black Death of the fourteenth century began in central Asia and Mongolia (where the plague bacillus remains endemic among rats, marmots, squirrels, and other small mammals). In the 1330s a warming of the climate in this region drove the animal plague hosts into closer proximity to humans. From human settlements and nomad camps, the plague spread along the trade routes linking central Asia with Black Sea ports that had been established by wealthy European cities such as Genoa and Venice. Ships traveling back to Europe from these ports helped transmit the disease westward.

The spread of the Black Death, however, was also in part a deliberate act of warfare—a medieval version of bioterrorism. The Tatars, a nomadic people of central Asia and the plains of Russia, staged frequent raids on trading caravans and...