The plague spread around port cities, as this was along trading lines which was where usually the plague was the most. The plague spread quickly along sea trade routes, it was abundant there.
The Black Death, also known as the bubonic plague was a very deadly pandemic. Around 25 million people died in total to where they had to burn bodies in roads and dump them on islands. Infected fleas were the culprit of the Black Death and there were a lot of them on ships and trade routes.
The Silk Road spread the plague around to lots of places, as infected rats were on the ships. Since the disease spread a lot in sea and trade routes, normally people in port cities would get infected and die.