Yersinia pestis is a bacterium that causes the infectious disease known as the plague. It primarily affects rodents but can be transmitted to humans through bites from infected fleas or direct contact with infected animals. The disease can manifest in several forms, including bubonic, septicemic, and pneumonic plague. First identified in the late 19th century, Yersinia pestis is responsible for several pandemics, including the infamous Black Death in the 14th century. Today, it is still present in some regions, particularly in parts of Africa, Asia, and the Americas, but modern antibiotics can effectively treat infections if caught early.