Sepsis is a dangerous systemic immune response that occurs in response to an infection. Sepsis is usually characterized by fever, high heart and breathing rate, and high white blood cell count. The infection that leads to sepsis is often (but not always) bacterial, and it may be localized (in one part of the body, e.g., the foot). However, “sepsis” is sometimes mistakenly used to mean the same thing as “septicemia”, which means the microorganisms causing sepsis can be found in the bloodstream.