How plague kills
The three forms of plague described in medieval writings correlate to the three ways in which plague bacteria may affect the body.
Bubonic plague occurs when the bacteria enter the body through the skin, as with a flea bite. The bacteria congregate in the lymph nodes of the neck, armpits, or groin, forming lumps called buboes, before spreading to other organs. Blood vessels, filled with bacterial waste, burst under the skin, creating dark blotches. Delirium and death usually follow.
Pneumonic plague, which kills within two or three days, occurs when a person inhales infected droplets coughed up by another victim. The bacteria invade the lungs, causing a bloody cough that can infect others in turn.
In septicemic plague, the bacteria enter through the skin but somehow bypass the lymph nodes, overwhelming the bloodstream directly. The victim suffers massive internal bleeding and sudden death within hours, before recognizable symptoms develop.
Based on "Apocalypse Then: A History of Plague." The 1992 World Book Health & Medical Annual, pp. 166-181.