that would be a stretch to say that Europeans in general got more resistance. The mutation offering more protection or even immunity did significanly spread in the gene pool, that is true. However, that would fail to explain why the outbreak in London (to give an example) in 1666 claimed so many dead.

If I recall correctly it is estimated that about 90% of the native american population died because of unknown diseases. However, some of those became endemic later on. Plague didn't (nor did that other scourge: smallpox). Plague is therefore one of those diseases that IF found by a doctor, must be reported to the national health authorities almost immediatly (as: within 24 hours) in my country.

The 1348 outbreak coincided with a period of bad harvests and relative overpopulation. Resulting in lower resistance among the populace. It was, in the words suitable for a five year old: a double whammy