Germs, there ya go. Rats almost always have fleas (and lice) even when there is no plague. Rats get the blood-borne disease, fleas suck their blood and hop to a human and infect the human when the flea back-flushes his siphon.

Humans in the sw USA get plague every year but it fails to spread, because we no longer have endemic flea populations in the human population. In the USA. If you live in the sw, make sure your pets don't have fleas.

B

PS Google Semmelweis (I think that's the spelling) for info on puerperal fever. That's the deathbed disease women once died from in alarming numbers and it was due to unclean doctors. Midwives were generally cleaner than doctors of the day and so, had less loss of patience due to that problem.