In the 1340s, Yersinia pestis made its way along the Silk Road to Europe.[13] The plague is believed to have re-entered the continent via Kaffa (now Feodosia), a trading station on the Black Sea that Genovese merchants had bought in the late thirteenth century from the khanate of the Golden Horde—part of the Mongol Empire that dominated the Western Eurasian Steppe. As the location of the region’s biggest slave market, where Genovese traders bought humans captured in the region north of the Black Sea and trafficked them to Europe, the Middle East and North Africa, Kaffa was one of the key links
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This situation, a small group of individuals come into contact with a larger population, seems to be the prefered way many pathogens spread.
Given the small communities in Europe (compared to Asia), the pathogen may have remained in Asia. If the Genovese had not come in contact with the various groups of people associated with the East, then Western Europe may have been spared.
Moreover, it could be said then that the Black Plague was caused by slavery.
