One thing about the black death of the middle ages is that it killed a smaller percentage of the population of europe than the diseases of europe and africa kill the populations of the americas. The black death killed maybe 60% of the population of Europe where as the 95@% of the population of the new world was killed by european and african diseases.
that suggested that europe already had some immunity to the plague. how did that happen. Scientists got their answer in the last couple years. Somewhere around 2800 bc the first black death visited europe. It also came from central asia. It wiped out large sections of the european population which was a combination of western hunter gatherers and anatolian farmers. Right after that steppe herders from modern ukraine moved in and over a 1000 years replaced all the men of europe. so that male with european ancestry traces his lineage back to these people.
first of all, Black death killed about third of the population in Europe. All the traditional historians and history records agree to this number. Anything more is a present day exaggeration. Media now like to exaggerate everything!
But Europe was always exposed to Asia and Africa, one way or other there were always travel routes active.
Population of Americas and Pacific islanders were isolated, so when first European arrived, the population was devastated.
The smallpox and other European diseases actually often preceded actual European visitors.
E.g. Peruvian Incas were already devastated by smallpox when Pizarro showed up.