One main reason why Poland escaped relatively unscathed, was the decision by Poland’s king, Casimir the Great, to close the country’s borders – and set up internal quarantines.
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Milan’s significantly lower mortality rate may also be down to the city’s stricter quarantine measures: The houses of infected families were simply bricked up (with the infected left to die inside).
Like Poland, the French-Spanish area, corresponding to the then-kingdom of Navarre, may have benefited from its relative isolation. Why the area around Bruges – then a thriving port with connections to the Mediterranean – might have been spared, is more of a mystery.

Frank Jacobs „Why did the Plague spare Poland?”