I had often heard that Poland was able to skip the first wave of the Black Death thanks to a swift shutdown of international trade. Is there any truth to this, or is it just propaganda? If it is true, how much of that has to do with centrally-imposed travel restrictions as opposed to the ravages of war?
There are many tellings of this myth, but that's what it is – a myth. Poland appears to have been less badly affected by the first wave of the Black Death c.1350 than was much of Europe to its west, but if so that experience had nothing to do with shutting down trade links. It was very badly hit by the second wave of the disease in 1360 and, overall, its experience of the pandemic was probably very similar in terms of severity to those of other European nations.
To understand how, when and why the story of Polish exceptionalism spread, you need to look further, as I did for a mega-post on the Polish experience of the Black Death that you can read here.
Hopefully this will answer all your questions as to this aspect of epidemiological history, but if not I stand read to address any follow-up questions you have.