D. Plague Plague can occur in two different clinical manifestations: bubonic plague and pneumonic plague. Bubonic plague is typically transmitted by infected fleas, not ticks. Yersinia pestis, the causative agent, enters the body of the victim as the flea is feeding. The bacteria are then transported to the local lymph nodes where they multiply and cause the characteristic swellings (bubo). Pneumonic plague can result when the bacteria spread from the lymph nodes via the blood stream to the lungs or directly by inhalation of aerosolized bacteria from a patient with pneumonic plague.