[Fever of unknown origin : horse, zebra or unicorn ?]

Despite the diagnostic means currently available, fever of unknown origin (FUO) remains a diagnostic challenge for the practitioner, requiring a detailed and systematic approach. The etiologies can be classified into 4 main groups: infectious diseases, systemic inflammatory diseases, neoplasia and various rarer entities such as drug-induced or endocrine fevers. A non-negligible percentage of FUOs remains without a clear etiology, generally with a spontaneously favorable evolution. In this article, we propose an algorithm for a step-by-step diagnostic approach and highlight clues that can lead to a diagnosis. However, clinical judgment remains the main pillar in the proposed approach in order to avoid futile investigations.