Black Death started in Mongolia in the 1200's. Mongolians spread it over Asia, later in time rats spread the disease around Asia killing 35 million people. It is thought to have originated from Yersinia Pestis, a bacterium.
This disease spread from fleas on rats. It spread and killed people in Asia. Finally the fleas got on the clothes of people traveling West. These people traveled on trade ships and once they arrived in Italy the disease spread. Later traveling to England, France, and Germany