There are three different types of plague: bubonic plague, septicemic plague, and pneumonic plague. The most common type is bubonic plague. Septicemic plague occurs when the Yersinia pestis bacteria (the organism responsible for the disease) multiply in the blood. Pneumonic plague is the most serious of the three types of plague. It occurs when plague bacteria infect the lungs, causing pneumonia.
"Famine, plague, and war are the three most famous ingredients of this wretched world…All animals are perpetually at war with each other…Air, earth and water are arenas of destruction." (From Pocket Philosophical Dictionary).