The source of plague in the “Plague Village” of Eyam in Derbyshire is believed to be from imported cloth contaminated with fleas.
https://www.eyamvillage.org.uk
Appreciate that info. Back then, transport of cloth, anything was not rapid-— took months. So, if it was fleas in the cloth, what were they eating? Fleas obligate blood suckers,maybe even back then they could eat crumbs or plant debris. Fairly sure that all this time, it has been... blood.
So, it was most likely rats feeding their fellow traveling fleas.