Biology info?

Bacteria were discovered and people became aware of disease spreading by bad habits and filth including fleas and rats and dirty water 

The likely explanation is just this: the Black Death was simply too deadly to persist. Evolutionary theory tells us that a pathogen that kills its victims will eventually run out of victims, leading to its own extinction. Combine that with how they figured out quarantining helped.

It didn't. It still makes the odd apperance.