Suggestion is that this plague was a major contributing factor for the regression into the Dark Ages of European History. Finally died out ca. 1710. Hosts Maintain the P. pestis by partially resistant individuals and their fleas. Colorado 9 OR 7 etc. By infected flea feces being scratched into skin.
Female flea requires a blood meal prior to ovogensis.
Currently a disease of 3rd world - undeveloped and unsanitized regions. And little fleas have lesser fleas, and so ad infinitum. Camus, A. 1947. Conniff, R. 1995. When it comes to the pesky flea, ignorance is bliss.