Final answer:
The plague was most likely spread to the European city of Caffa by flea-infested rats. It reached Caffa in 1347 through ships carrying these plague-bearing rats. From Caffa, the plague spread to Italy and eventually throughout Europe along trade routes and ports.
Step-by-step explanation:
The plague was most likely spread to the European city of Caffa by flea-infested rats. Caffa was an important administrative center of Genoese trade with a major port that served as a hub of merchant activity. The plague reached Caffa in the summer of 1347 when plague-bearing rats boarded ships from the city. It then spread from Caffa to Italy and eventually throughout Europe along trade routes and ports.