I thought it was infected fleas entering sailing vessels in Black Sea ports and exiting the vessels in Mediterranian and Altlantic ports.
The rats moving down the lines from ships to piers were observed and an account survived, but A) that’s unsurprising as observations go, and B) the writer didn’t note that the rats stopped to scratch themselves. :^)
Human crews probably contributed to the introduction and spread.
https://health.wyo.gov/wyoming-detects-rare-human-case-of-pneumonic-plague/