In 1346, a siege at the Genoese port of Caffa became an unlikely vector for the Black Death to reach Europe and devastate the Golden Horde. This episode explores how Yersinia pestis, carried by fleas on rats, traveled along the Silk Road from Central Asia, killing tens of millions and weakening the Jochid Ulus beyond recovery. We examine the controversial theory that the Mongols may have used biological warfare during the siege, a claim debated by historians like Samuel Kohn. Beyond the battlefield, we look at how the pandemic triggered the Great Troubles, a period of fragmentation that allowed Moscow to rise while Sarai declined. You will hear about Khan Uzbek’s earlier tolerance of Christianity, the role of the darughachi in tax collection, and how the sudden death of key leaders accelerated the Horde’s collapse. This story connects microbiology with geopolitics, showing how a microscopic organism reshaped the map of Eurasia.