[Fever of unknown origin in the 21st century: infectious diseases].

Fever of unknown origin (FUO) is a rare but important disease. The definition of FUO has not changed in the last 50 years. Classical FUO is defined by an illness of at least 3 weeks duration with fever greater than 38 masculine C, and no established diagnosis after 1 week of hospital investigation. The causes of FUO can be divided in four categories: infectious diseases, noninfectious inflammatory diseases, neoplasms, and others (miscellaneous). Recent studies have surprisingly shown that despite improved diagnostic procedures the percentage of patients with FUO, in which no diagnosis after intensive investigations in specialized centres can be found, has increased. However, finding the correct diagnosis in FUO is essential for these patients for psychological and vital reasons. Therefore and because of economic reasons patients with FUO should be investigated in specialized centres with a department for rheumatology and infectious diseases.