There is a good book called “Justinians Flea” (cannot remember the author) regarding the emergence of the plague in Constantinople. The premise is that rats and their fleas arrived in Constantinople on grain ships from Egypt. The plague decimated the population of Constantinople. Rats being opportunistic hitchhiked on ships and other transportation to all corners of Europe creating the pandemic we call the “Black Death”.
It is amazing how fast disease spread in the early years of civilization with primitive shipping and overland caravans.
I saw a video a while back about the spread of the Spanish Flu from Europe back to the US and Canada via returning WW I soldiers. They came home on ships and immediately boarded trains to get to their homes throughout the US and Canada. The spread was incredibly fast. The first transcontinental railroad in North America was completed only 50 years earlier and very quickly became a vector for fast disease transmission.