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 is characterized by a bubo, i.e. a swelling of the lymph node draining the flea bite site. 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. Initial symptoms of bubonic plague appear 7–10 days after infection. 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.