First, let’s understand fever and the body’s thermoregulatory mechanism: Under normal conditions, the body maintains a dynamic balance between heat production and heat loss. Fever occurs when heat production increases or heat loss decreases due to various causes. In most patients, fever is due to thermogenic sources, which include exogenous and endogenous sources. (1) Exogenous pyrogens: such as microbial pathogens and their products, inflammatory exudates, sterile necrotic tissues, antigen-antibody complexes, etc., cannot act directly on the thermoregulatory center, but through activation of neutrophils, eosinophils and monocyte-phagocyte system in the blood to produce and release endogenous pyrogens, causing fever by the following mechanisms; (2) Endogenous pyrogens: also called (2) endogenous pyrogens: also known as leukocyte pyrogens, such as interleukin (IL-1), tumor necrosis factor (TNF), and interferon. Through the blood-cerebrospinal fluid barrier, they act directly on the thermoregulatory center of hypothalamus, which makes the thermoregulatory center (temperature threshold) rise, and the thermoregulatory center must re-regulate the body temperature to send out impulses, and through the pituitary endocrine factors to increase metabolism or through the motor nerve to make the skeletal muscle contraction (clinical manifestation of chills), so that heat production increases; on the other hand, through the sympathetic nerve to make the skin vascular and vertical hair muscle contraction sweating Stop, heat loss is reduced. This combined effect of regulation makes heat production greater than heat dissipation, and the body temperature rises to cause fever. In addition, there are also non-thermogenic fever, which can be caused by: (1) direct damage to the thermoregulatory center, such as cranial trauma, hemorrhage, inflammation, etc.; (2) diseases that cause excessive heat production, such as epilepsy, hyperthyroidism, etc.; (3) diseases that cause reduced heat dissipation, such as extensive skin disease, heart failure, etc. Definition of fever to be investigated: Fever of more than 2 to 3 weeks duration, with body temperature above 38.5℃, which is not clearly diagnosed by detailed and complete etiological inquiry, physical examination and routine laboratory tests, is called Fever Occasion unknow (Fou), which can last for several months or more than a year, and is often a clinically disturbing problem. Doctors should have an idea of the causes of fever of unknown origin and the frequency of its occurrence. These “three categories” summarize the etiology of more than 80% of patients. It is generally believed that infection accounts for 40%, neoplastic diseases for 20%, connective tissue – vascular diseases for 15%, various other diseases for 15%, and those whose causes are always unknown for 7-10%.