Fever of unknown origin (FUO, aka pyrexia of unknown origin, PUO, febris e causa ignota, febris ECI) is a condition where the patient has a fever (elevated temperature), but despite Ix by a doctor, no explanation has been found.
Localized pyogenic infections
Systemic bacterial infections
Other bacterial infections
Systemic rheumatic and autoimmune diseases
Inherited and metabolic diseases
Dx of exclusion, i.e. eliminating all possibilitie suntil only 1 explanation remains, taking this as the correct one