Black Death: 75-200M (1334-1353)
The second pandemic of the bubonic plague was initiated in north-eastern China. It killed around five million, and quite fast. Then, it moves west, through India, Syria, and Mesopotamia.
In 1346, it struck a trading port known as Kaffa in the Black Sea. Ships that were departing from there carried trade goods and rats that also carried fleas, which had Yersinia pestis.
It sounds like a long chain of unuseful information, but trust me, it makes sense. In October 1347, 12 of these ships docked at Messina in Sicily, and they were full of sick sailors.
By the time harbor authorities actually had the chance to understand what the ships brought, it was way too late. In the following five years, the Black Death killed half of Europe.
Panic stirred a fervor for scapegoating, and anti-Semitic groups feared the rapid advance of the epidemic. Some towns, like Marseilles, gained reputations as safe havens.
For a short period of time, wars halted and trade stopped altogether. Cropland went to seed. Wages rose with increasing demand for labor, and chinks of opportunity for real social change opened up.
