Purpura fulminans is an acute, often fatal, thrombotic disorder which manifests as blood spots, bruising and discolouration of the skin resulting from coagulation in small blood vessels within the skin and rapidly leads to skin necrosis and disseminated intravascular coagulation.

Purpura fulminans is caused by defects in the protein C anticoagulant pathway. Identification of the cause of purpura fulminans often depends on the patient's age and circumstances of presentation.

Congenital (inherited) defects in protein C activity are autosomal dominant and may be partial or severe loss of function. Hundreds of natural mutations of the protein C gene (PROC) have been identified.

Acquired protein C deficiency is caused by either depletion of available protein C in plasma or decreased protein C synthesis (caused by administration of vitamin K antagonists, severe liver failure or complications of prematurity).

Purpura fulminans is a presenting feature of severe acute sepsis, including cases caused by Neisseria meningitidis, Streptococcus pneumoniae and Group A or B streptococcal infections, and less commonly with Haemophilus influenzae, Staphylococcus aureus, Capnocytophaga canimorsus or Plasmodium falciparum (malaria) infections, particularly in individuals with asplenia.

In some cases, a combination of sepsis and a partial congenital defect in the protein C anticoagulant pathway initiates purpura fulminans.

In rare instances, purpura fulminans is an autoimmune manifestation against protein C or protein S after normally benign infections, such as chicken pox. Sometimes purpura fulminans has unknown cause.

Regardless of the underlying cause of purpura fulminans, the mechanism of disease is similar with deficiency in protein C concentration or decrease in protein C activity which promotes blood clotting (thrombosis).