What really caused the bubonic plague?

The bubonic plague, also known as the Black Death, was caused by the bacterium _Yersinia pestis_.

_Yersinia pestis_ is a gram-negative, rod-shaped bacterium that is primarily transmitted by fleas. The bacteria live in the gut of fleas, and when the fleas bite a human, the bacteria are regurgitated into the human's body.

Once in the human body, the bacteria can cause a variety of symptoms, including fever, chills, swelling of the lymph nodes (buboes), and skin lesions. In severe cases, the bacteria can also cause death.

_Yersinia pestis_ is thought to have originated in Central Asia, and it spread throughout the world along trade routes. The most famous outbreak of the bubonic plague occurred in Europe in the 14th century, where it killed an estimated 75-200 million people. The disease continued to cause periodic outbreaks in Europe and Asia until the 19th century.

Today, the bubonic plague is still a disease of some concern, although it is much less common than it was in the past. Most cases of the bubonic plague occur in rural areas of Africa, Asia, and South America. The disease can be treated with antibiotics, and the prognosis is generally good if treatment is started early.