Introduction Fever is typically a symptom of an identifiable infection or illness. However, in some cases, fever persists without an obvious cause despite initial evaluation. This is known as Fever of Unknown Origin (FUO). FUO presents a diagnostic challenge, requiring a systematic approach to identify underlying conditions, which may include infections, inflammatory disorders, malignancies, or rare diseases. Early recognition and careful evaluation are critical to ensure timely and appropriate treatment. Definition of FUO Traditionally, FUO is defined as: Note: Definitions may vary slightly based on guidelines, but the core concept remains a prolonged, unexplained fever. Common Causes of FUO While…