The Black Death, an infamous outbreak of plague, was caused by the bacterium Yersinia pestis. The plague originated in the Gobi Desert in late 1320s. By 1347, the disease reached Europe, where it decimated the continent. Roughly one-third to one-half of the population died from the plague.
The Black Death was primarily a form of plague called bubonic plague. In this form, Y. pestis causes black lumps in the skin called "buboes." Other symptoms included fever, headaches, chills and tender lymph nodes. Before modern medicine, the disease usually meant a quick but painful death. During the Black Death episode of plague, the disease spread so rapidly and killed so many that people believed it was the end of time.
Y. pestis can live in mammals and fleas. During the Black Death, rats acted as a reservoir for the bacterium. The plague originally entered Europe via ship rats in the trading port of Genoese, or Genoa, in Italy. Plentiful rats allowed the disease to spread along trade routes throughout Europe. It spread so rapidly that it reached Britain within a year.
In most cases, bubonic plague cannot spread from directly mammal to mammal. However, fleas can transfer the plague between rats and act as a vector to humans. Y. pestis can survive longer in a flea's body than a mammal's. The flea acquires the bacterium when it makes a blood meal of an infected rat or human, then the flea spreads it to the next creature it bites.
The bacterium that caused the Black Death still exists. In the Southwestern United States, the bacterium lives in many rodents, including mice, prairie dogs and rabbits. Hunters occasionally contract plague from their kills. Additionally, pets can bring plague into the home, either from infectious fleas or the more dangerous airborne version of the plague, called the pnuenmoic plague.