They fled Central Asia in 1347 in hopes to escape the Black Death, they made their way to Europe with no idea that the cause of the disease was aboard the ship with them.
Rats infested with fleas had climbed aboard the ship before they had set sail across the Mediterranean sea. The fleas feed off of the rats blood during their journey towards Europe.
The ship then made a stop at Sicily (an island by the boot of Italy) but did not stay for very long, but it was long enough for the rats to escape the boat and make their way onto land.
Many rats stayed on the ships and continued to travel aboard the ships until the got off and roamed through the cities and towns of Europe.
As the rats roamed the streets of Europe the fleas jumped from the rodents to humans causing the rats blood to enter the humans blood stream
The rats that stayed on board also infected the members aboard the ships.
In 1351 the bugs causing this plague mainly died off causing the deaths to cease. This disease resulted in the death of around 75 to 200 million people. In 1894 a person called Yersin found the cause of the bacteria and made a cure.