Background
Plague is a bacterial disease caused by Yersinia pestis, which primarily affects wild rodents. It is spread from one rodent to another by fleas. Humans bitten by an infected flea usually develop a bubonic form of plague, which produces the characteristic plague bubo (a swelling of the lymph node). If the bacteria reach the lungs, the patient develops pneumonia (pneumonic plague), which is then transmissible from person to person through infected droplets spread by coughing. If diagnosed early, bubonic plague can be successfully treated with antibiotics. Pneumonic plague, on the other hand, is one of the most deadly infectious diseases; patients can die 24 hours after infection. The mortality rate depends on how soon treatment is started, but is always very high.