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.

• Infection:
  • Localized pyogenic infections
  • Intravascular infections
  • Systemic bacterial infections
  • Mycobacterial infections
  • Other bacterial infections
  • Rickettsial infections
  • Chlamydial infections
  • Viral infections
  • Fungal infections
  • Parasitic infections
• Neoplasm:
  • Hematologic malignancies
  • Solid tumors
  • Benign
• Noninfectious inflammatory:
  • Systemic rheumatic and autoimmune diseases
  • Vasculitis
  • Granulomatous diseases
  • Autoinflammatory syndromes
• Inherited and metabolic diseases
• Thermoregulatory disorders
• Habitual hypothermia
• Miscellaneous

• Dx of exclusion, i.e. eliminating all possibilitie suntil only 1 explanation remains, taking this as the correct one