The establishment of the Mongol khanate resulted in the deaths of tens of millions of people (40 million is a common guess). There was some recovery in population once the empire was in place, and new opportunities opened up for trade across the Eurasian steppe. But, just as with the earlier trade between Rome and China, there were also new opportunities for microbes to spread. The Black Death probably killed even more of the world’s population than the Mongols. Western Europe, spared Mongol invasion, lost perhaps a third of its population to the plague in the fifteenth century.
In China, the plague probably struck Mongols even worse than Chinese, and coincided with the overthrow of Mongol rule and establishment of a native dynasty. The new dynasty, the Ming, was more authoritarian than the native Sung dynasty that preceded Mongol rule. The Sung state got most of its revenues from taxes on trade, internal and external, and was solicitous of mercantile interests. The Ming returned to the more traditional practice of getting most of its revenues from taxing the peasantry; it returned as well to the traditional Confucian distrust of merchants.
In Eastern Europe, two states did well during this period: the Ottoman sultanate and Poland. The Ottomans expanded into both Anatolia and the Balkans. And Poland, which had been defeated, but not subjugated, by the Mongols, mostly avoided the plague for some reason. It would go on to occupy a huge chunk of Eastern Europe. But below the level of states and empires, something else was going on. Aristocracies in Eastern Europe responded to the loss of population by intensifying serfdom, binding peasants ever more firmly to their estates. Eventually the “second serfdom” east of the Elbe would be far more intense than the first serfdom of the medieval West had ever been.
In Western Europe by contrast, the loss of population in the Black Death helped to end serfdom. At first, European aristocrats, like their eastern counterparts, tried to prevent workers from taking advantage of the law of supply and demand. The Statute of Laborers in England (1349-51) complained that
The servants, having no regard … but to their ease and singular covetousness, do withdraw themselves from serving great men and others, unless they have livery and wages double or treble of what they were wont to take … to the great damage of the great men and impoverishment of all the commonalty.
The Statute forbade servants and small-holders from taking higher wages. But these efforts largely collapsed by the end of the century, partly thanks to the economic and political clout of West European cities. Aristocrats would continue to hang onto their lands and rents, but serfdom largely disappeared.