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.