We verified patient diagnoses and retrospectively analyzed clinical manifestations, physical signs, laboratory examinations, and treatments. These analyses were completed in combination with literature review and collection of follow-up diagnosis and treatment information for analysis; follow-up was accomplished by means of outpatient contact, telephone, and the use of a mobile phone app. FUO was defined as: fever that lasted ≥3 weeks; temperature >38.3°C ≥2 times; and no diagnosis even after evaluation of complete disease history, physical examination, and routine laboratory tests for ≥1 week.