Living conditions in the big cities of the Mongolian Empire were no better than in the Holy Roman Empire. Mediæval cities literally stunk from all the pee and poop. Wherever you get a lot of people living closely together you’ll also find rats. Why not? Rats gotta eat and there’s always kitchen trash around wherever people are. The good thing was: rats got rid of a lot of garbage by eating it. The bad thing was: rats had fleas. The worse thing was: rats and fleas carried bubonic plague bacteria.
People who didn’t take baths much had fleas, too. A flea who’d bitten a rat with the plague might jump onto a human being and bite him. By the mid-1300s, Mongolian traders carried fleas and plague bacteria east and west along the Silk Road. Plaguey rats and their fleas hitched rides on grain-laden caravans. Everywhere traders went, they spread bubonic plague.
Study hard, pay attention in class and you may land a job looking at 2,000-year-old turds: https://www.ancient-origins.net/news-history-archaeology/2000-year-old-feces-silk-road-reveal-spread-infectious-diseases-006326
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