ABSTRACT
The first work on prolonged fevers was by Keefer, entitled Prolonged and Perplexing Fevers; it was the first compilation of infectious and noninfectious disorders characterized by prolonged fevers (1). The term fever of unknown origin (FUO) was first used by Petersdorf in 1961 to define prolonged febrile illnesses that were not easily diagnosed. Petersdorf defined FUO as an obscure fever with a duration 3 weeks, temperatures1018F (38.38C) on multiple occasions during three weeks, remaining undiagnosed after a week of hospitalization. These FUO criteria have remained useful (2-5).
