SIRS, or systemic inflammatory response syndrome, describes the clinical definition of inflammation of the body with two or more symptoms including a fever of over 100.4, a heart rate elevated over 90 beats per minute, a respiration rate of over 20 per minute, or an abnormal white cell count. Sepsis is SIRS PLUS a source of infection.

All sepsis meets SIRS criteria, but not the converse. SIRS criteria may be met by other etiologies such as dehydration, trauma or ischemia. These processes may raise heart rate, respiratory rate, white count and sometimes fever, and therefore meet SIRS criteria, but are not infections. So early on, it is not always clear whether a patient has sepsis, or some other process causing SIRS criteria.