This episode tackles the diverse etiologies of Fever of Unknown Origin (FUO) in adults, based entirely on the comprehensive content from UpToDate. It defines FUO as a prolonged fever without an obvious cause after extensive evaluation and organizes its causes into five main categories: infections, rheumatologic conditions, neoplasms, miscellaneous causes, and undiagnosed cases. The podcast explores how epidemiology varies by geography, patient age, and even climate change’s impact on infectious causes, with examples like tuberculosis, adult-onset Still’s disease, cancers, and conditions such as sarcoidosis or factitious fever. Each subtopic is summarized with the key phrase, “if you can’t remember everything, remember this,” to help residents retain the essential diagnostic insights needed to navigate this challenging clinical puzzle.