Fever is a regulated increase in body temperature (more than 0.5°C) caused by an upward shift of the thermoregulatory point due to the action of a pyrogen. Normal body temperature varies slightly from person to person and is influenced by many factors (time of day, season, environment, menstruation, etc.). Therefore, the best way to determine whether or not you have a fever is to compare your body temperature with your usual temperature under the same conditions. If you do not know your original body temperature, an axillary temperature (10 minutes of testing) of more than 37.4°C is considered feverish. Fever is caused by the action of fever activators on the body, which leads to the production of endogenous pyrogens (EP) and enter the brain to act on the thermoregulatory center, which in turn leads to the release of mediators from the thermoregulatory center and then causes a change in the regulatory point, and ultimately causes fever. Common fever activators include exogenous pyrogens from outside the body, such as bacteria, viruses, fungi, spirochetes, and malaria parasites; and endogenous pyrogens from inside the body, such as antigen-antibody complexes and steroids. Endogenous thermogenesis (EP) comes from the body’s EP-producing cells, the main types of which are: interleukin-1 (IL-1), tumor necrosis factor (TNF), interferon (IFN), interleukin-6 (IL-6), etc. EP acts on the thermoregulatory center located in the POAH, resulting in the production of both positive and negative regulatory mediators. The latter can cause a change in the point of regulation and ultimately lead to the development of fever. Fever itself is not a disease, but a symptom. In fact, it is one of the body’s mechanisms for fighting infection. Fever may even have its uses: to shorten the duration of illness, to increase the effectiveness of antibiotics, and to make the infection less contagious. These abilities should counteract the discomfort experienced during a fever. Diagnosing the cause of Fever of Unknown Origin (FUO) is a worldwide challenge, with nearly 10% of FUO cases never having a clear cause. Fever itself can be caused by a variety of diseases, such as infections, tumors, autoimmune diseases, and blood disorders, and cannot be clearly categorized. In the past, these patients were usually seen by internists, but in most hospitals with more specialized departments, they were mainly seen by respiratory physicians. Nowadays, many hospitals have set up departments of infectious diseases and put FUO under the department of infectious diseases, and this kind of specialized management is a kind of progress, which can improve the level of diagnosis and treatment.There is a precise definition of FUO, which contains three points: ① fever duration ≥3 weeks; ② temperature >38.3℃ many times; ③ after ≥1 week of complete history questioning, physical examination, and routine laboratory tests, the diagnosis still cannot be confirmed. It can be seen that although FUO itself is a symptom diagnosis, not a disease diagnosis, the diagnostic requirements are very strict.