Interesting. The main stream documentary I was watching concluded it was fleas and lice more than rats.
Rats can be carriers of both. I remember one of my history teacher ssaying that there was something about horses that repelled those fleas and ticks, which they didn't know at the time, but the people who were hired to cart away the dead bodies slept in the stables with their horses and were supposedly immune to the plague by virtue of whatever the immunity that was granted to them by being in close proximity to their horses all the time.
Fleas and lice can ride around on rats.
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