Yersinia pestis is a highly infectious and deadly bacterium that causes the plague, also known as the Black Death. There are several synonyms used to describe this pathogen, including Pasteurella pestis, Bacillus pestis, and Yersinia pestis biovar medievalis. Each of these names refers to the same bacterial species, which is transmitted to humans through the bite of infected fleas or contact with infected animals. The disease caused by Yersinia pestis has long been feared for its ability to spread rapidly and cause widespread death and devastation. Despite advances in modern medicine, the plague remains a serious global health threat and is closely monitored by public health officials.