Like some other form of gram-negative sepsis, septicemic plague can cause disseminated intravascular coagulation, and is almost always fatal without treatment (the mortality rate in medieval times was 99-100 percent). Septicemic plague is the rarest of the three plague varieties; the other forms are bubonic and pneumonic plague.
Transmission and mode of action 
The disease is contracted primarily through the bite of an infected rodent or insect, but like bubonic plague can very rarely be contracted through an opening in the skin or by cough from another infected human. After initial infection, the bacteria multiply in the blood, causing bacteremia and severe sepsis. In septicemic plague, bacterial endotoxins cause disseminated intravascular coagulation (DIC), where tiny blood clots form throughout the body, possibly resulting in ischemic necrosis (tissue death due to lack of circulation/perfusion to that tissue).
DIC results in depletion of the body's clotting resources, so that it can no longer control bleeding. Consequently, there is bleeding into the skin and other organs, leading to red and/or black patchy rash and hemoptysis/hematemesis (respectively coughing up or vomiting up of blood). There are bumps on the skin looking somewhat like insect bites; these are usually red, and sometimes white in the center.
Untreated, septicemic plague is usually fatal. Early treatment with antibiotics reduces the mortality rate to between 4 and 15 percent. People that contract this disease must receive treatment in at most 24 hours, or death is almost inevitable. In some cases, people may even die on the same day they contract it.
The usual symptoms are:
- Abdominal pain
- Bleeding due to blood clotting problems
- Low blood pressure
- Organ failure
However, septicemic plague may cause death before any symptoms occur.
Septicemic plague in medieval times 
Septicemic plague was the least common of the three plague varieties that occurred during the Black Death from 1348 to 1350 (the other two being bubonic plague and pneumonic plague). Like the others, septicemic plague spread from the East through trade routes on the Black Sea and down to the Mediterranean Sea.
Major port cities such as Venice and Florence were hit the hardest. The massive loss of working population in Europe following the Black Death, resulting in increased economic bargaining power of the serf labour force, was a major precipitating factor for the Peasants' Revolt of 1381.
See also 
- Medline Plus - Plague, NIH, retrieved 2011-03-24
- History Learning Site - Black Death of 1348 to 1350, History Learning Site, retrieved 2011-06-06