1. Prolonged unexplained fever (PUO) is defined as a fever above 38.3°C for at least 3 weeks without a confirmed diagnosis after outpatient visits or 3 days of hospitalization. 2. Infections are the most common cause of PUO (45% of cases), followed by malignancy (20%) and connective tissue diseases (15%). 3. Common infectious causes include tuberculosis, endocarditis, hepatitis, and infections of the abdomen, urinary tract, bones/joints, and dental/sinus areas. Viral, bacterial, fungal, and parasitic infections can also cause PUO.