Severity of illness (SOI) is defined as the extent of organ system derangement or physiologic decompensation for a patient. It gives a medical classification into minor, moderate, major, and extreme. The SOI class is meant to provide a basis for evaluating hospital resource use or to establish patient care guidelines.
It is possible to consider "severe illnesses" those that have “major” and “extreme severity”
Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on May 2, 2018.