"From a genomic perspective, the majority of post-Black Death strains of Yersinia pestis thus far identified in Europe display diversity accumulated over a period of centuries that form a terminal sub-branch of the Y. pestis phylogeny. It has been debated if these strains arose from local evolution of Y. pestis or if the disease was repeatedly reintroduced from an external source."

https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1011404