An illness of at least 3 weeks’ duration with fever exceeding 38.3°C on several occasions and diagnosis not established after 1 week of hospital investigation. The main causes are systemic and localized infections, neoplasms, or collagen-vascular diseases (e.g., rheumatoid arthritis, disseminated lupus erythematosus, and polyarteritis nodosa). Less common causes are granulomatous disease, inflammatory disease of the bowel, pulmonary embolization, drug fever, cirrhosis, and rare conditions such as Whipple’s disease. Diseases such as AIDS, chronic fatigue syndrome, or Lyme disease are occasionally the cause of FUO. Some cases remain undiagnosed.