fever of unknown origin (FUO), a febrile illness of at least 3 weeks’ duration with a temperature of at least 38.3° C (100.9° F) on at least three occasions and failure to establish a diagnosis in spite of intensive inpatient or outpatient evaluation (three outpatient visits or 3 days’ hospitalization). The duration of febrile illness required to establish a diagnosis of FUO varies among authorities and is sometimes given as shorter than 3 weeks.
