The rapid transfer of people and goods across Central Asia spread more than ideas and inventions. It also spread diseases, the most deadly of which was the plague known in Europe as the Black Death, which scholars identify today as the bubonic plague. In the early fourteenth century, transmitted by rats and fleas, the plague began to spread from Central Asia into West Asia, the Mediterranean, and western Europe. The confusion of the mid-fourteenth century that led to the loss of Mongol power in China, Iran, and Central Asia undoubtedly owes something to the effect of the spread of the plague and other diseases. (For more on the Black Death, see Chapter 14.)

Traditionally, the historians of each of the countries conquered by the Mongols portrayed them as a scourge. Among contemporary Western historians, it is now more common to celebrate the genius of the Mongol military machine and treat the spread of ideas and inventions as an obvious good. There is no reason to assume, however, that people benefited equally from the improved communications and the new political institutions of the Mongol era. Merchants involved in long-distance trade prospered, but those enslaved and transported hundreds or thousands of miles from home would have seen themselves not as the beneficiaries of opportunities to encounter new cultures but rather as the most pitiable of victims. Moreover, the places that were ruled by Mongol governments for a century or more — China, Central Asia, Persia, and Russia — do not seem to have advanced at a more rapid rate during that century than they did in earlier centuries, either economically or culturally.

In terms of the spread of technological and scientific ideas, Europe seems to have been by far the main beneficiary of increased communication, largely because in 1200 it lagged farther behind than the other areas. Chinese inventions such as printing, gunpowder, and the compass spread westward. Persian and Indian expertise in astronomy and mathematics also spread. In terms of the spread of religions, Islam probably gained the most. It came to dominate in Chinese Central Asia, which had previously been Buddhist.