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.