There are three primary forms of plague, caused by Yersinia
pestis which is transmitted via fleas: bubonic plague
, septicemic plague and pneumonic plague.
Bubonic plague accounts for about 80-90 percent of cases, with septicemic accounting for 10-20 percent. Pneumonic plague is rare. Overall, estimated mortality for untreated plague is 60-100 percent; less than 15 percent if treated.