Fever of unknown origin (FUO) and inflammation of unknown origin (IUO) are syndromes that share a similar spectrum of causes and that are separated by an arbitrary temperature threshold [1]. They group together febrile and inflammatory disorders that persist for more than 3 weeks and remain unexplained despite a thorough workup [2]. Those with recurrent FUO/IUO, defined by the presence of at least one fever/inflammation-free interval, are often considered to be the most elusive subgroup [3–5]. Older studies indicated a high number of miscellaneous disorders and undiagnosed patients in the recurrent group [3,5].
