Sepsis is defined as the development of a systemic inflammatory response syndrome (SIRS) as a result of an infective process. SIRS is characterized by two or more of the following conditions:

The above definitions of these different syndromes were made at a consensus conference jointly sponsored by the American College of Chest Physicians and the Society for Critical Care Medicine in 1991.

Severe sepsis may progress to multiple organ dysfunction syndrome (MODS), defined as organ dysfunction caused by systemic inflammation-induced damage in which homeostasis cannot be maintained without supportive measures. This may take the form of a primary insult (e.g. aspiration injury to the lung) or a secondary phenomenon (e.g. adult respiratory distress syndrome (ARDS) in severe pancreatitis). MODS was first described more than 20 years ago, following the observation that some patients succumbed to progressive organ system failure despite an initial phase of apparent stability. The phenomenon is a dynamic process which varies from patient to patient and even within the same patient at different times.

Despite better monitoring and supportive techniques in the intensive care unit (ICU), the mortality rate associated with sepsis remains high, especially when accompanied by shock and MODS.