Medically unexplained fever is defined as a fever that lasts more than 3 weeks, has a temperature of more than 38.3°C, and remains undiagnosed after 3 days of hospitalization or 3 outpatient visits. Unexplained fever must be promptly hospitalized to identify the etiologic agent before the etiology can be treated. Many clinically unexplained fevers may be associated with major or serious diseases, such as tumors, AIDS, tuberculosis, immuno-rheumatic diseases, and other diseases. The causes of fever are diverse and are generally divided into infectious and non-infectious fevers. Infectious fever is often caused by infection due to invasion of the organism by bacteria, viruses, fungi, mycoplasma, chlamydia, rickettsia, etc. For bacterial infections, such as respiratory infectious diseases pneumonia, lung abscess and abscess chest, the pathogen can be clarified clinically by doing blood routine, calcitoninogen and bacterial culture, etc. The treatment is mainly anti-inflammatory treatment with antibiotics, and the fever can be controlled only after the inflammation is eliminated, and for drug-resistant bacterial infections, effective antibiotics need to be selected for anti-inflammatory treatment in combination with drug sensitivity test. Tuberculosis infection can also cause long-term fever (such as pulmonary tuberculosis, intestinal tuberculosis, etc.). The clinical manifestation of fever in tuberculosis infection is usually low fever in the afternoon, and X-ray, CT examination or tuberculin test can help to make the diagnosis. For non-infectious fever, such as some malignant tumors, immune diseases, rheumatism, etc., the cause must first be identified before symptomatic treatment can be given. So for unexplained fever, one must not blindly go for antipyretic drugs. Because the blind use of antipyretic drugs may cover up the condition and lead to misdiagnosis and delayed treatment, which not only cannot effectively control the body temperature, but also increases the risk of liver and kidney damage to the body from antipyretic drugs. Therefore, it is recommended to use physical cooling before the cause is identified, and it is not advisable to use antipyretic drugs blindly.