According to Wikipedia, it’s lice that are the vector, not fleas.
The rats carry the fleas OR lice. The fleas or lice carry the bacteria/virus that kills.
California had a mini outbreak of the plague WAY back when the Chinese first came to the "golden mountain" to work on the railroads and in the mines. There were a few, very few rodents who got infected. Luckily they were destroyed.
Even the rough and ready California miners weren't as FILTHY as the mainstream Chinese in China, so the plague didn't spread.
The plague was a product of unremitting FILTH which was part and parcel of the Chinese culture. Still is, as far as I could see (and smell). I don't CARE what their government says...one can smell the stench of sewage in their rivers.