A lot of rats and fleas aren't carriers anymore it basically exists in a smattering of regions at this point. Fun fact every once in awhile people in the Southwestern US get infected usually either by bad luck or the occasional insane hippie eating a rat raw, no I am not joking that happens once in awhile. But yeah humans, rats, and fleas were basically in a weird infection ouroboros so when humans broke that chain it helped reduce it massively.

Also fun fact the plague that may have lead to Indo-Europeans migrating into Europe proper was probably spread through human to human contact since rats weren't all that much of a factor at the time.