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

there is a difference between the normal one and the one in the lungs (that's 99% lethal).

The intensity depends on two other factors:

1. response of local authorities (which over the course of the centuries became more adept at dealing with outbreaks: one such measure was quarantaining (The Venetians had an island for that), ships using special flags to signal the disease on board, etc. The duke of Milan infamously bricked in any house that had a patient in it: harsh but it worked.

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2. the health of the population: as mentioned before, resistance did improve in the population, HOWEVER: famines (either due to war or bad harvests) impacted the health of the population that was affected by an outbreak.

It didn’t, there were several so-called echo epidemics after that one. Later on, the disease did reoccur locally, with varying degrees of intensity