The bacterium responsible for causing plague is Yersinia pestis. It is a Gram-negative rod that primarily infects rodents and is transmitted to humans through flea bites. Yersinia pestis causes different clinical forms of plague, including bubonic, septicemic, and pneumonic plague, characterized by swollen lymph nodes (buboes), septicemia, and severe pneumonia respectively. The high virulence and rapid progression of infection make early identification and treatment critical. Other options listed cause different diseases and are not associated with plague.

Reference: Mandell, Douglas and Bennett's Principles and Practice of Infectious Diseases, Mandell GL et al., 9th Edition.