I wrote out paragraphs to try and explain it too. And I come to the same conclusion. I put it this way:

>Think about it. Humans are built to spread pathogens by breath. Especially in a world where the literacy rate is in the single digits, and there is no mass communication.

News traveled by word of mouth.

You don't even need the fleas at that point. You just need panicked people running from the plague in every direction. Some of whom will be carriers.

It's really not a mystery at all.