Sepsis: Commonly called a "blood stream infection." The presence of bacteria (bacteremia) or other infectious organisms or their toxins in the blood (septicemia) or in other tissue of the body. Sepsis may be associated with clinical symptoms of systemic (bodywide) illness, such as fever, chills, malaise (generally feeling "rotten"), low blood pressure, and mental status changes. Sepsis can be a serious situation, a life threatening disease calling for urgent and comprehensive care.