Pneumonic plague, a severe type of lung infection, is one of three main forms of plague, all of which are caused by the bacterium Yersinia pestis. It is more virulent and rarer than bubonic plague. The difference between the versions of plague is simply the location of the infection in the body; the bubonic plague is an infection of the lymphatic system, the pneumonic plague is an infection of the respiratory system, and the septicemic plague is an infection in the blood stream.