Definition of Fever of Unknown Origin

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Fever of unknown origin: The presence of fever greater than 38.3'C (101'F) 'off and on' for more than three weeks without a specific cause identified for the fever. Abbreviated FUO. Extensive diagnostic testing can determine the cause in the majority of cases of FUO. FUO may be related to infections such as HIV or other viral infections, cancers, or chronic inflammatory diseases such as sarcoidosis.