Severity of illness (SOI) is defined as the extent of organ system derangement or physiologic decompensation for a patient. It denotes a medical classification as either minor, moderate, major, or extreme. The SOI classification is meant to provide a basis for evaluating hospital resource use or to establish patient care guidelines.

It is possible to consider "severe illnesses" as those with “major” and “extreme severity.”

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on May 2, 2018.