The organism which, reputedly, causes plague [it was called Pasteurella pestis when I taught microbiology] is comparatively fragile outside of a living body. Doubtful if it could survive hundreds of years.
On the other hand, if the plague affected the northern parts of Scandinavia and Russia there might well be some victims buried in the permafrost. Odds [IMO] would be much higher to recover some of the caustive entity in this case.