What causes bubonic plague and how is the plague transmitted ?

Bubonic plague is the most common form of plague in the wild. It most often follows infection by the bite of infected fleas from a rat or other small mammal, but it can also be done by exposure to biological fluids of a dead animal infected with the plague. It is one of the three types of plague caused by the bacteria Yersinia pestis.

The plague is first declared in rodents, bitten by infected fleas, they die quickly. Fleas losing their host seek other sources of blood, and contaminate humans or pets. In human epidemics, it is the human flea that takes over. In the fourteenth century, a pandemic of bubonic plague, called Black Death crossed Europe, causing the death of a third to half of its population.