Plague (Yersinia pestis) Technical Description: Plague Close Window Plague, caused by the Gram-negative bacterium Yersinia pestis, can describe two major disease processes: bubonic plague, spread by fleas and maintained in an animal reservoir (rats); and pneumonic plague, which is spread by the inhaled route and is of most relevance to the modern biological warfare (BW) threat. In the classical form of bubonic plague, fleas take blood meals from rodents (e.g., Rattus rattus) infected with the Yersinia pestis bacteria. Following ingestion of blood from the plague-infested animal, the foregut (proventriculus) of the flea becomes blocked with plague bacteria. Without the ability to digest nutrients, the flea desperately looks for any host to sate its appetite. At this point, the flea attempts to discharge the contents of the foregut as it bites, allowing plague bacteria to enter the wound. While the bacteria are picked up by macrophages in the host, these often fail to adequately deal with the bacteria, and these spread to regional lymph nodes. Thus, bubonic plague is often associated with the raised, swollen bubos at the groin. Symptoms include darkened areas of tissue at the extremities, thus the moniker “Black Death” for the classical disease. Plague bacteria acquired in this manner can seed the lungs, causing an infectious, pneumonic form of plague. Ultimately, death from plague is due to respiratory failure and/or collapse of the circulatory system. When transmitted person-to-person in the respiratory (pneumonic) form of illness, progression to disease is faster, probably due to the enhanced virulence of the organism from the primary host. In primary pneumonic plague, Yersinia pestis bacteria can be inhaled via infectious respiratory droplets, or as aerosolized agent in a BW context. The incubation period for insect-transmitted plague is approximately two-eight days, and one-three days in the pneumonic form. Left untreated, bubonic plague has a mortality rate of about 50%, while pneumonic plague is almost always fatal. Treatment of pneumonic plague with antibiotics (doxycycline, ciprofloxacin, and/or chloramphenicol) can be successful, but it is necessary to administer chemotherapy within at least one day of the onset of symptoms.