What Caused the Black Plague to End?

The black plague (or Black Death) was spread by a bacteria that lived on fleas. Those fleas in turn lived on the black rat. As the population of the rats spread, so did the disease. After several years of this, a new species of rat, the brown rat, appeared in the area. The brown rats did not carry the infected fleas and soon the population of the black rat dwindled to nothing, which virtually eliminated the bacteria carrying fleas.