https://en.wikipedia.org/wiki/Yersinia_pestis
~~~~~
Y. pestis is a facultative anaerobic parasitic bacterium that can infect humans primarily via its host the Oriental rat flea
~~~~~
The Holy Roman empire was composed of grain farming communes. As with all grain farming communes HRE had rats in abundance.
Occasional recent diagnoses in the US have been related to dogs and cats kept as pets, same wheelhouse.