Also, this might have something to do with it: from what I understand, O blood type is uncommonly… common in Poland. Something to do with large families in small villages and a LOT of intermarriage. The black plague was caused by a bacterium that produced, in its waste in the human body, wastes that very closely mimic the “B” marker sugars on red blood cells that keep the body from attacking its own immune system. Anyone who has a B blood type had an immune system that was naturally desensitized to the presence of the bacterium, and therefore was more prone to developing the disease. Anyone who had an O type was doubly lucky because the O blood type means the total absence of ANY markers, A or B, meaning that their bodys’ immune system would react quickly and violently against the invaders, while someone with an A may show symptoms and recover more slowly, while someone with B would have just died. Because O is a recessive blood type, it shows in higher numbers when more people who carry the recessive genes marry other people who also carry the recessive gene. Poland, which has a nearly 700 year history of being conquered by or partnering with every other nation in the surrounding area, was primarily an agricultural country, focused around smaller, farming communities where people were legally tied to, and required to work, “their” land, and so historically never “spread” their genes across a large area. The economy was, and had been, unstable for a very long period of time leading up to the plague, the government had been ineffective and had very little reach in comparison to the armies of the other countries around for a very very long time, and so its people largely remained in small communities where multiple generations of cross-familial inbreeding could have allowed for this more recessive gene to show up more frequently. Thus, there could be a higher percentage of O blood types in any region of the country, guaranteeing less spread of the illness and moving slower when it did manage to travel. Combine this with the fact that there were very few large, urban centers where the disease would thrive, and with the above facts, and you’ve got a lovely recipe for avoiding the plague.
Interestingly enough, as a result from the plague, the entirety of Europe now has a higher percentage of people with O blood type than any other region of the world.
