Despite the continuous improvement of modern medical equipment and new methods of diagnosis and treatment, fever of unknown origin (FUO) is still a daunting challenge for medical practitioners. The medical criteria for fever of unknown origin (FUO) are: 1) fever lasting ≥3 weeks; 2) temperature >38.3℃ several times; 3) diagnosis cannot be confirmed after ≥1 week of complete medical history, physical examination and routine laboratory tests. There have been many reports on unexplained fever at home and abroad over the years, and the diseases that may cause unexplained fever may vary from season to season, region to region, and era to era. However, most studies believe that there are three main categories of diseases causing unexplained fever: infectious diseases, tumors, and autoimmune diseases. Infectious diseases are the most common, accounting for about 60-70% of all unexplained fevers; tumors and autoimmune diseases each account for about 10-15%; and some patients are ultimately difficult to diagnose, accounting for 10-20%. Among infectious diseases, tuberculosis is the most common; among autoimmune diseases, vasculitis is the most common; among tumors, lymphoma is common before the age of 65, and gastrointestinal tumors are common after the age of 65.