Bubonic plague is a disease that, in the Middle Ages, wiped out a third of humanity because of poor sanitation. This "black death" began in Mongolia and China caused by a bacterium known as Yersinia pestis. This bacteria thrived on its host, the rodent. The rodent easily snuck onto cargo ships which sailed to markets throughout Europe and Asia. These ships dispersed the bacteria where it would creep through populations and kill them quickly.

This map illustrates the pattern of bubonic plague's spread at the beginning of the deadliest pandemic. It traveled by sea because of the ships and spread a little bit inland, depending on economic activity and terrains. 

 

The Black Death killed 75 million people in Western Europe. In London, 18,000 people died in three years. Worldwide, there were 100 to 200 million deaths. The mortality rate of the disease is 30% - 75%.  It was the worst outbreak of all time.