The plague swept Eurasia because they took items from Africa, brought it to China, and then, from China, it was brought to Europe. And the traders kept bringing it, until there were thousands of cases.
Answer:
The trade
Most historians today generally agree that the plague was likely spread through Eurasia via these trade routes by parasites carried on the backs of rodents. ... The disease spread along the active trade routes that northern Italian and Flemish merchants had developed.
