Septicemic (or septicaemic) plague is one of the three main forms of plague. It is caused by Yersinia pestis, a gram-negative species of bacterium. Septicemic plague is a life-threatening infection of the blood, most commonly spread by bites from infected fleas.
Like some other forms of gram-negative sepsis, septicemic plague can cause disseminated intravascular coagulation, and is almost always fatal when untreated (the mortality rate in medieval times was 99-100 percent).[citation needed] However, it only occurs in a minority of cases of Yersinia infection, so that fewer than 5,000 people a year acquire the disease. It is in fact the rarest of the three plague varieties; the other forms are bubonic and pneumonic plague.