Fever of Unknown Origin (FUO) By Larry M Bush, MD, FACP, Charles E Schmidt College of Medicine, Florida Atlantic University Reviewed / Revised Aug 2022 | Modified Sep 2022 View Patient Education Etiology Evaluation Treatment Geriatrics Essentials: FUO Key Points Topic Resources Some Causes of Fever of Unknown... Fever of unknown origin (FUO) is body temperature ≥ 38.3 ° C (≥ 101° F) rectally that does not result from transient and self-limited illness, rapidly fatal illness, or disorders with clear-cut localizing symptoms or signs or with abnormalities on common tests such as chest x-ray, urinalysis, or blood cultures.