FUO is currently classified into 4 distinct categories: Classic FUO: Fever for > 3 weeks with no identified cause after 3 days of hospital evaluation or ≥ 3 outpatient visits Health care-associated FUO: Fever in hospitalized patients receiving acute care and with no infection present or incubating at admission if the diagnosis remains uncertain after 3 days of appropriate evaluation Immune-deficient FUO: Fever in patients with neutropenia and other immunodeficiency if the diagnosis remains uncertain after 3 days of appropriate evaluation, including negative cultures after 48 hours HIV-related FUO: Fever for > 4 weeks in outpatients with confirmed HIV infection or > 3 days in inpatients with confirmed HIV infection if the diagnosis remains uncertain after appropriate evaluation Etiology of FUO Causes of FUO are usually divided into 4 categories (see Table: Some Causes of Fever of Unknown Origin (FUO)): Infections (25 to 50%) Connective tissue disorders (10 to 20%) Neoplasms (5 to 35%) Miscellaneous (15 to 25%) Infections are the most common cause of FUO.