Abstract: Fever is a common clinical manifestation of many diseases and is one of the common reasons for patients to see a doctor. However, the diagnosis of the cause of fever in many patients is very difficult, sometimes even unable to find the exact cause. Fever is also known as fever of unkown origin (FUO), It was defined FUO as body temperatureabove 38.3°C on three or more occasions and a duration of illness of at least three weeks, in which no diagnosis was made after one week of hospital admission or physical examination and routine laboratory examination. The concept of fever to be investigated provides an important clinical idea for the diagnosis and treatment of patients with unexplained fever. In recent years, with the progress of medical technology and the change of epidemiology, the spectrum of febrile diseases has changed correspondingly. This article aims to discuss and summarize the composition of fever, distribution and change trend of disease spectrum, and provide a reference for clinical diagnosis.

Key words: Fever of unknown origin, Disease spectrum, Changes