Black Death was virulent and deadly. As such, everyone got it and you either lived (unlikely but possible) or died (most likely) By the middle of the 14th century, pretty much everyone in its path was a survivor or a victim. The disease had nowhere further to spread to.

As you can see, by 1351, Black Death had been EVERYWHERE in or around Europe. It was too early for people go to to the Americas, and the Orient is where the disease came from. There was no-one left to infect

That included the vector for the disease, the black rat.

The plague was deadly to rats too, and as the disease spread to a population that hadn’t had it before, the black rat started to be displaced by the Norwegian rat, which didn’t carry the disease.

Plague still exists of course, but since it was so successful just about everyone alive today is directly descended from someone who survived it.

It’s still virulent, but now the mortality rate is way down. In England, one entire village survived exposure without a single case or death, and a study of Ugandan prostitutes who never got AIDS showed they had something in common – a type of T-cell.

Plague, like AIDS, attacks the cells of the immune system, and the more T-cells you have, the easier it is for the disease to survive. In this genetic variation (which is now more widespread), your T-cells are more resistant to being infected themselves.