I agree with you that this question deserves proper investigation, and that this sub needs a better sourced answer to what is a pretty frequently posted query. My contribution is this short critical review of sources. I need to start by stressing that I do not read Polish and that the real answer, insofar as one does exist or can exist, is almost certainly going to be found in specialist economic and demographic studies done by Polish-speaking scholars. I'd strongly encourage anyone able to supplement our knowledge with information drawn from such sources to do so.

That said, I think we can make a couple of basic points that take us further than we've gone so far. The first is that Poland has not always been associated with low incidences of plague deaths. Something happened at some point in the 20th century to change our ideas as to how the relative incidence of plague in Poland should be viewed, and it's interesting and revealing to investigate what exactly that "something" was, and to what extent it was based on heavy-duty primary source research in Poland.

My second point is that there is some reason to suppose that the association of Poland with low incidence of plague is a product not of any actual variance in the death rate, but of lack of sources or lack of research. Ziegler, in his influential popular history of the Black Death (1969), makes the point that "until quite recently it was accepted tradition that the plague scarcely penetrated to Castile, Galicia and Portugal"; but he accepts that more detailed investigation of these regions dispelled the idea. This seems important to me, especially as discussions of Poland as mysteriously almost "plague free", and forming a dramatic contrast to the rest of Europe, are pretty much entirely an internet phenomenon; while historians of the period, and even historians of plague, concede that the region did experience the Black Death somewhat differently to the lands to the west, they do not see that history as so different that they seem to have felt compelled to launch in-depth investigations to understand why; the explanation most commonly suggested in scholarly texts is that the population density of the area in this period was too low to allow the disease to be transmitted readily. Hence it seems to me to be at the very least possible that the idea that the Polish experience of the Black Death was unique, or almost so, is actually wrong, and that the explanations you cite for the supposed lesser incidence of plague deaths in the region (no cat-killing, the leadership of Casimir the Great) are post-hoc rationalisations - attempts to find explanations that fit the supposed evidence - rather than being sourced in contemporary materials.

Finally, I have to wonder to what extent lack of contemporary source materials impacts on the difficulty of discussing what happened in Poland during the plague years. Assessing the number of deaths caused by the Black Death is a notoriously difficult thing to do, even in countries with significant quantities of extant records, with estimates for total mortality varying dramatically from author to author; even today, after decades of intensive research, one can read estimates of mortality across Europe that vary from a third up to a half up to 60 percent of total population, so it would be astonishing if there was anything like a consensus for Poland, and indeed the materials I've read suggest there are dramatic instances of disagreement on crucial matters such as the approximate population of Poland (and even what constituted the "borders of Poland") in the period 1000-1500. Until such matters are resolved (if it is even possible to do so), we will never be in a position to make any statements about the demographic impact of the Black Death in Poland with any confidence.

Part I: A survey of histories of the plague in Poland, 1832-2015

OK, let's start with a survey of what's been said about the impact of the Black Death in Poland.

• Hecker, a once highly renowned German medical historian, says in his On the Black Death (1832), that the plague arrived in Poland from Germany, and cites the work of Jan Długosz - better known to English speakers as Johannes Longinus Dlugloss – in Annales seu cronici incliti regni Poloniae(c.1480) as his authority for the statement that "in Poland the infected were attacked with spitting of blood and died in a few days in such vast numbers that... scarcely a fourth of the inhabitants were left" - that is, he thought mortality in Poland was practically 75%.

• Most other works published around the middle of the nineteenth century mention Poland only in the context of what was then seen as the most interesting and remarkable thing about the plague's passage through eastern Europe: "it had thus made the circuit of the Black Sea, by way of Constantinople, southern and central Europe, England, the Northern Kingdoms, and Poland, before it reached Russian territories, a phenomenon which has not again occurred with respect to more recent pestilences originating in Asia." (Buckle's Common Place Book 556 (1872)

• Sticker, in his monstrous Treatises on the History of Epidemics and Episcopal Doctrine (1908/10), basing himself on two contemporary chronicles, suggests that the Black Death entered Poland from Hungary and caused incredible mortality. He writes that the plague killed half the population and depopulated entire towns and villages. This estimate is in line with the contemporary figure normally given for plague deaths elsewhere in Europe.

• Similarly, the Polish Encyclopædia (1921) states that "long, for instance, did the land bear the traces of the 'Black Death'... which swept down with unabated fury upon Poland after decimating the populations of Italy, France, England and other countries of western and central Europe."

• This seems to have been the consensus for some time. As late as 1969, George Deaux, in his The Black Death, 1347, could write that "the Black Death attacked Hungary and Poland at the same time as it appeared in Austria and with the same results. Towns were left totally depopulated..." In this context, it is interesting to note that Deaux (a popular writer) had access to no population studies for the region more recent than Hecker's and makes the same claims about 75% mortality in Poland.

Then we come to the mid-20th century shift, after which it is generally accepted that something unusual did happen in Poland. Thus for example...

• Perry Anderson's Lineages of the Absolutist State, a very broad survey first published in 1974, observes that "Poland suffered less from the late feudal crisis than any other country in Eastern Europe; the Black Death (if not ancillary plague) largely passed it by, while its neighbours were ravaged."

• Joseph Strayer's Dictionary of the Middle Ages (1982) says that the plague was devastating "in all of eastern Europe save Poland."

• Norman Davies's God's Playground: A History of Poland (1982) states not only that "Poland escaped the scourge of the Black Death," but also makes the extraordinary claim that "economic life was not disrupted."

• And by 1983, a Rutgers professor, Robert S. Gottfried, could confidently state that "Poland lost about a quarter of its population to the plague."

To the extent that this "something" is defined, it's generally placed, as I mentioned above, in the context of relative population density – Poland was too sparsely populated to allow the plague to spread as rapidly as it did elsewhere. Sometimes the relative absence of trade – and hence of both travellers and the sort of transport of goods in which infected rats might hitch rides – is also suggested. It has also been argued that the relatively slow and late enserfment of peasants in eastern Europe suggests there was no demographic catastrophe in the region in the mid-14th century.

And the change in position has certainly been well-cemented:

• Paweł Jasienica's 1985 survey of Piast Poland only glances at the plague, but states that "the epidemic was less severe in Poland than elsewhere."

• Lukowski & Zawadzki's A Concise History of Poland (2005) suggest that "the Black Death left a sparsely populated Poland largely unscathed".

• Adam Zamoyski's popular Poland: A History (2009) goes so far as to state that "most of Poland remained unaffected. The populations of England and France, of Italy and Scandinavia, of Hungary, Switzerland, Germany and Spain were more than halved. Poland's grew, perhaps as a consequence of conditions elsewhere."

• But these are survey works, mostly interested in later periods of Polish history, in which flimsy research and sweeping statements may be expected. Robert Frost provides more nuanced discussion. He similarly suggests that the "relatively sparsely peopled lands of east central Europe did not suffer as badly as western and southern Europe from the Black Death," but adds that "Poland was not untouched. Population growth slowed, but its upward trajectory was not reversed, and by the mid fifteenth century the disparity between the density of settlement in Poland and western Europe had ceased."

Frost goes on to state that the stream of German migrants heading east was "abruptly halted" by the plague, and that this caused the Polish economy to collapse after 1350, in part because of shortages of specie flowing from the west. This may well be true, but, if so, it would partially mask any impact on Polish population and economic activity caused by the Black Death. I think this is an important point, since once we start to consider Poland as an interconnected part of the broader socio-economic history of eastern Europe, we necessarily also have to start wondering about any claims to either its "isolation" or its "uniqueness", both of which are central to the popular view of Poland as being mysteriously unaffected by the plague.