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Etiologies of fever of unknown origin in adults

David H Bor, MD
Section Editor
Peter F Weller, MD, MACP
Deputy Editor
Anna R Thorner, MD


Clinicians commonly refer to a febrile illness without an initially obvious etiology or without localizing signs as fever of unknown origin (FUO). This usage is not accurate. Most febrile illnesses either resolve before a diagnosis can be made or develop distinguishing characteristics that lead to a diagnosis. FUO refers to a prolonged febrile illness without an established etiology despite intensive evaluation and diagnostic testing.

Large case series of FUO applying this definition have been collected over a number of decades; these facilitate an approach to patients with FUO and an understanding of the changing patterns of FUO with time and newer diagnostic techniques.

The common and uncommon entities causing FUO in adults will be reviewed here. The definitions of this condition, an approach to the adult with FUO, and the etiology of FUO in children are discussed separately. (See "Approach to the adult with fever of unknown origin" and "Fever of unknown origin in children: Etiology".)

Three general categories of illness account for the majority of "classic" FUO cases and have been consistent through the decades:


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Literature review current through: Nov 2017. | This topic last updated: Apr 25, 2016.
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