Fever of unknown origin (FUO) and inflammation of unknown origin (IUO) are two clinical syndromes that are separated by an arbitrary temperature threshold. They share a common spectrum of underlying causes, which are typically categorized as infectious disorders, malignancies, noninfectious inflammatory disorders, and miscellaneous disorders [1]. The most recent definition defines FUO as an illness with a 3-week duration, fever of ≥38.3 °C on several occasions, and uncertain diagnosis despite a predefined work-up [2].
