One strain of the Black Death, the pneumonic plague, is a mutation of a standard pneumonia bacteria. Studies have shown that just a few important changes in the genetic code of this bacteria need to be made in order for the disease to become much more dangerous. This article from the Scientific American discusses research which has compared the genetic code between these two bacteria. There are two primary changes to the original bacteria that make it easier to be carried by fleas as a vector, and also reduce blood clotting in the victim in order to reduce resistance for the bacteria to enter the body.
In the 1300s, the Black Death killed many Europeans. The rapid proliferation of this epidemic is largely due to the mutations that lead to the pneumonic plague. An account from a man who survived the plague in Florence describes both the symptoms of the plague as well as how fast it spread throughout the city. He describes how everyone feared interacting with plague victims because so many people had been infected simply for treating those who had already had the plague.
Very few people contract the plague each year in the United States, and modern antibiotics can treat the plague quite effectively, but the plague can spread very easily, and could potentially get out of control if not properly treated.