Fevers of unknown origin (FUOs) are classified according to the underlying disorder. The 4 main clinical categories of FUOs are infectious, malignant, rheumatic/inflammatory, and miscellaneous disorders.
Although malignancy remains the most common cause of FUOs, rheumatic/inflammatory disorders remain important diagnostically and therapeutically. Rheumatic/inflammatory disorders, for example, systemic lupus erythematosus (SLE) presenting as FUO, have become uncommon in recent years because of better serologic diagnostic tests.