Fever of unknown origin (FUO), refers to a condition in which the patient has an elevated temperature (fever) but despite investigations by a physician no explanation has been found.[1][2][3][4][5]

If the cause is found it is usually a diagnosis of exclusion, that is, by eliminating all possibilities until only one explanation remains, and taking this as the correct one.

Causes

Extrapulmonary tuberculosis is the most frequent cause of FUO.[2] Drug-induced hyperthermia, as the sole symptom of an adverse drug reaction, should always be considered. Disseminated granulomatoses such as tuberculosis, histoplasmosis, coccidioidomycosis, blastomycosis and sarcoidosis are associated with FUO. Lymphomas are the most common cause of FUO in adults. Thromboembolic disease (i.e. pulmonary embolism, deep venous thrombosis) occasionally shows fever. Although infrequent, its potentially lethal consequences warrant evaluation of this cause. Endocarditis, although uncommon, is another important etiology to consider. Bartonella infections are also known to cause fever of unknown origin.[6]

Here are the known causes of FUO.[7]
https://en.wikipedia.org/wiki/Fever_of_unknown_origin