Definition of Fever: The normal body temperature of a healthy person was determined in the 19th century.Carl Reinhoid August Wunderlich took nearly 1 million axillary temperature measurements on 25,000 people.The average temperature was 37.0 °C, with a fluctuating range of 36.2 to 37.2 °C. The average temperature was 37.0 °C, with a fluctuating range of 36.2 to 37.2 °C. The average temperature was 37.0 °C. It was lowest at 6 a.m. and highest at 4 to 6 p.m. Generally, it is considered that the oral temperature is higher than 37.3 ℃, the anal temperature is higher than 37.6 ℃, or the change of body temperature in a day is more than 1 or 2 ℃ is considered fever. Fever within 2 weeks of the fever is called acute fever, acute fever patients with short fever, accompanied by obvious accompanying symptoms, etiological diagnosis is generally not difficult. Fever lasts for more than 3 weeks, the body temperature exceeds 38.3℃ many times, after at least 1 week of in-depth examination can not be diagnosed a group of diseases is called the cause of unknown origin fever (Fever of unknown origin, FUO). This is an important group of diseases, due to its complex etiology, often lack of characteristic clinical manifestations and laboratory findings has become a very challenging problem in medical practice. A body temperature (oral temperature) of 37.5~38.4℃ for more than 4 weeks is called prolonged low-grade fever, which has its own specificity clinically. Grading of fever: low fever: 37.3~38.0℃; medium fever: 38.1~39.0℃; high fever: 39.1~41.0℃; ultra-high fever: above 41.0℃. Mechanism of fever: it has been proved that the body temperature regulation center in the hypothalamus. The anterior part of the hypothalamus and the preoptic area around the presence of dense temperature receptors and a few cold receptors, heat production and heat response can be caused by stimulating this part (the anterior part of the hypothalamus is not a heat center); and the posterior part of the hypothalamus may be the neural “intelligence” to be integrated into the processing of the site (not a heat production center). The body’s thermoregulatory center regulates the processes of heat production and heat dissipation through neural and humoral factors in order to maintain the dynamic balance of body temperature. Skeletal muscles and liver are the main sites of heat production when the body is quiet, and even more so when exercise or illness is accompanied by fever; the body’s main site of heat dissipation is the skin, and about 90% of the heat is lost through radiation, conduction, convection, and evaporation. If these areas are dysfunctional due to various factors, it can lead to fever. Pyrogene (pye means “fire” in Greek) is a general term for a group of substances that can cause an abnormal increase in body temperature in thermostatic animals, and trace amounts can cause fever. Exogenous pyrogens (e.g. viruses, mycoplasmas, chlamydia, rickettsiae, spirochetes, bacteria and their toxins, fungi, protozoa, antigen-antibody complexes, pyrogenic steroids (e.g. urotestosterone), uric acid crystals, etc.) mostly do not pass the blood-brain barrier, but are mainly transmitted through the host cells (mainly macropinocytes and macrophages). The so-called endogenous pyrogens (e.g., IL-1, IL-6, IFN-α, IFN-β, TNF, etc.) produced by host cells (mainly macrophages and monocytes) act on the thermoregulatory centers and cause fever. However, there are exceptions: LPS can both act directly on the hypothalamus and prompt host cells to synthesize a variety of endogenous pyrogens. Thus, the etiology of fever is very complex and diverse. The purpose of fever is to increase the inflammatory response, inhibit bacterial growth, and create a pathophysiologic environment that is not conducive to the development of infection or other disease. Fever can be one of the common clinical manifestations of many types of diseases and is a common challenge in the diagnosis of infectious diseases. The common and rare causes of fever are listed as follows: i. Causes of infection in FUO. (i) Bacterial causes of (limited) inflammation: 1. Abscess; 2. Diverticulitis; 3. Endocarditis; 4. Cholestasis; 5. Implant infections; 6. Infected aortic aneurysm; 7. Catheter infections; 8. Osteomyelitis; 9. Genito-urinary sensation; 10. Dental and ear, nose and throat infections, etc. (ii) Other (systemic) bacterial infections: 1. Spirochetal disease; 2. Brucellosis; 3. Cat scratch disease; 4. Nosocomial anthrax; 5. Whipple’s disease; 6. Mycobacteriosis; 7. Tuberculosis; 8. Atypical mycobacteriosis; 9. Psittacosis; 10. Q-fever; 11. Salmonellosis; 12. Yersinia pestis; and others. (iii) Viral infections: 1. EBV infection; 2. HIV infection; 3. Cytomegalovirus infection, etc. (D) Fungal infections: 1, Aspergillosis; 2, Candidiasis; 3, Histoplasmosis; 4, Cryptococcosis; 5, Pneumocystis carinii infection. (v) Parasitic infections: 1. Amoebiasis; 2. Malaria; 3. Toxoplasmosis; 4. Visceral leishmaniasis (black fever) etc. II.Non-infectious etiologies in FUO. (i) Hematologic neoplasms: 1. Lymphoma; 2. Leukemia; 3. Myelodysplastic syndromes, etc. (II) Solid tumors: 1. Lung cancer; 2. Hepatocellular carcinoma; 3. Colon cancer; 4. Renal cell carcinoma; 5. Pleural mesothelioma, etc. (iii) Connective tissue and inflammatory vascular diseases: 1, Felty (Felty’s syndrome); 2, Hypersensitivity vasculitis; 3, Ankylosing cristitis; 4, Leukoaraiosis; 5, Nodular polyarteritis; 6, Recurrent polychondritis; 7, Giant cellular vasculitis/rheumatic polymyalgia; 8, Dermatomyositis; 9, Schnitzler’s syndrome (measles-vasculitis); 10, Systemic lupus erythematosus; 11, adult systemic Still’s disease; 12, Takayasu’s vasculitis; 13, Wegener’s granuloma, etc. (iv) Other causes in FUO: 1, angioimmunoblastic lymphadenopathy; 2, drug fever; 3, Castleman syndrome; 4, inflammatory pseudotumor of lymph nodes; 5, exogenous anaphylactic alveolitis (hypersensitivity pneumonitis); 6, familial Mediterranean fever; 7, hyper IgD syndrome; 8, idiopathic granulomatous disease (including granulomatous hepatitis); 9, Crohn’s disease; 10 , necrotizing lymphadenitis; 11, cryptogenic hematoma; 12, mesenteric lipomatosis; 13, retroperitoneal fibrosis; 14, recurrent pulmonary embolism; 15, tuberculosis; 16, subacute thyroiditis; 17, phytophagia; 18, pseudo-fever, self-induced fever; 19, atrial mucocele; 20, cyclic neutropenia, and so on. It can be seen that fever is not a small problem, but a great deal of learning, so when encountering fever, do not want to save trouble, oral antipyretics, especially for more than a week, it is recommended to the hospital for examination and treatment. On the application of antipyretics views have not been unified. It is believed that antipyretics will change the heat type, affecting the diagnosis and prognosis of the judgment as well as affecting the evaluation of the effect of treatment, some drugs can also affect the patient’s defense function, such as aspirin can inhibit interferon, prolonging the viral shelling; salicylic acid can reduce the survival rate of experimental animals. However, emergency cooling measures should be taken for hyperthermia, postoperative hyperthermia, hyperthermic delirium, and hyperthermia in infants and children. Antipyretics should be used with caution, as a sudden drop in body temperature accompanied by profuse sweating can lead to collapse or shock. The elderly and the frail should pay particular attention to. 39 ℃ or more physical cooling can also be used as an emergency cooling measures, cooling effect of alcohol, warm water bath is particularly commonly used, ice or cold water bag placed on the forehead, armpits, groin cold compresses can also be attempted, but the latter effect of the cooling effect is slightly worse; conditions, while lowering the room temperature (so that the room temperature is maintained at 27 ℃ or so), the effect of cooling is more desirable. I hope that we all pay attention to fever, avoid drug abuse, hide the development of the disease, delayed diagnosis.