Sepsis is a bacterial infection that causes a systemic inflammation of the body and a host of other immune system responses. The systemic inflammation ironically occurs from an overwhelming release of cytokines and other mediators, which exist to fight infection. These cytokines and mediators may cause the blood vessels to dilate, lowering blood pressure, sometimes to the point that the blood cannot properly perfuse the body, causing a state known as septic shock. When tissues that do not receive adequate blood flow, they frequently release an additional substance into the blood, lactic acid, which may be found on blood testing and aid in the diagnosis of septic shock.

In septic shock, blood may also begin to clot inappropriately which, in combination with low blood pressure, may further deprive major organ systems (such as the brain, kidneys, and liver) of adequate blood flow. Failure of one system may lead to failure of other systems, a state known as multiorgan system failure.