Normal adult body temperature is generally about 36 to 37℃, oral temperature (sublingual temperature) 36.3 to 37.2℃, rectal temperature (anal surface temperature) is generally 0.3 to 0.5℃ higher than oral temperature, and axillary temperature is 0.2 to 0.4℃ lower than oral temperature. Normal body temperature varies slightly between individuals and often fluctuates due to internal and external factors, for example, body temperature is slightly higher in the afternoon than in the morning, slightly higher in hot environments, after strenuous exercise, labor or meals, lower in the elderly than in young adults, and slightly higher than normal in women before menstruation and during pregnancy. When the body in the role of pyrogen or various reasons caused by the thermoregulatory center dysfunction, body temperature rises beyond the normal range, it is called fever. Taking the oral temperature as the standard, 37.3~38℃ is low fever, 38.1-39℃ is moderate fever, 39.1-41℃ is high fever, and over 41℃ is super high fever. We should not be unfamiliar with fever. Fever caused by common pathogenic infections such as common cold, acute gastroenteritis, purulent tonsillitis, etc. is easily diagnosed, and the body temperature can soon return to normal after various anti-infection treatments. However, sometimes the situation is not as simple as we think. There is also a group of diseases where the fever lasts for more than 3 weeks and the temperature exceeds 38.3℃ several times, and the diagnosis is still not clear after at least 1 week of intensive examination, which is clinically known as fever of unknown origin (FUO). Patients often go to multiple hospitals and use multiple medications, but their temperature remains unchanged. According to statistics, 40% of all febrile illnesses under investigation are caused by various infections, 30% by connective tissue diseases, 20% by neoplastic diseases, and 10% are ultimately undiagnosed. Although the etiology of fever is extremely complex, careful history taking, careful physical examination, necessary and sometimes repeated laboratory tests and auxiliary examinations can confirm the diagnosis of 90% of the etiology. 1. History questioning and fever pattern When you go around to many hospitals, you may get bored and feel that the doctor’s history questioning is similar and you have repeated yourself several times. However, perhaps your patient and careful recollection of the circumstances of your illness, what triggers there were, whether you had contact with ticks, parrots, other animals, whether you had been to an infected area, what accompanying symptoms there were, what the fever pattern was, whether the fever could be reduced without medication, how long it took for the fever to reduce with medication, etc., will give the doctor many diagnostic clues. Different fever patterns can also suggest different causes, such as fever with a constant temperature of 39-40℃ for several days or weeks, which is common in lobar pneumonia, typhus and typhoid fever; flaccid fever with a temperature above 39℃ and fluctuations of more than 2℃ within 24 hours, but above normal levels, which is common in sepsis, rheumatic fever, severe tuberculosis and septic inflammation, infective endocarditis. Intermittent fever in which the body temperature rises to a peak and then falls to normal within a few hours, then rises again after a day or several days, is common in malaria, acute pyelonephritis, and biliary tract infection; bimodal fever in which the body temperature rises and falls twice in a day is seen in gram-negative bacillary sepsis; undulating fever in which the body temperature rises gradually to 39℃ or more, then falls gradually to normal after several days, then rises gradually after several days, is common in brucellosis, connective tissue disease, tumor, etc. It is common in brucellosis, connective tissue disease, tumor, etc.; regressive fever in which the body temperature rises abruptly above 39℃, then drops abruptly to normal after a few days, and the high fever period and the fever-free period each last for a few days and then alternate regularly, is common in regressive fever spirochete infection, Hodgkin’s lymphoma, periodic fever, etc.; irregular fever is seen in tuberculosis, rheumatic fever, bronchopneumonia, exudative pleurisy, etc. For example, enlarged temporal artery may be temporal arteritis, conjunctival petechiae may be subacute endocarditis, enlarged lymph nodes may be lymphoma, tuberculosis or cytomegalovirus infection, etc. Red or hemorrhagic skin spots on palms and soles (Janeway damage), red nodules on fleshy parts of fingers (toes) ( Osler nodules) are seen in infective endocarditis, a transient polymorphic rash suggests still disease, and a hemorrhagic rash and molting on the palms of the hands and feet is seen in systemic lupus erythematosus. Therefore, the doctor may not spare the nail bed, lymph nodes, anus and vulva, etc. Some signs may be transient, and you should remember to tell the doctor. 3. Laboratory auxiliary examinations are crucial Laboratory examinations and auxiliary examinations are of great significance in diagnosis. Some examinations need to be sent repeatedly to improve the detection rate or infer the diagnosis from the progress of the disease; some examinations are invasive, such as bone marrow aspiration, bone marrow biopsy, lymph node biopsy, skin biopsy, liver biopsy, etc., and the results can provide very important diagnostic clues; some examinations use new technology and new equipment. Some tests use new technology and new equipment, which are more expensive, such as PET-CT, which has a strong suggestive effect on the discovery of tumor lesions. Doctors generally do not easily propose invasive or expensive tests, but if needed, it is recommended to perform them as early as possible to obtain diagnosis and start treatment as soon as possible. For patients with fever whose diagnosis is not yet clear, the use of glucocorticoids to reduce fever is generally not advocated, because hormones will not only mask the symptoms, but also aggravate or induce infection; nor is the abuse of multiple antibiotics advocated, because the long-term use of multiple antibiotics will not only cause a huge economic waste, but also make the positive rate of pathogenic tests such as bacterial culture drop greatly, and even lead to drug fever and secondary infection. The use of antipyretic agents to cool the body should also be prudent, because antipyretic agents will change the heat type, affecting the diagnosis, sudden drop in body temperature with a lot of sweating may lead to deficiency or shock, can use alcohol bath, ice cold wet compresses and other physical methods to cool. Among the infectious diseases accounting for 40% of the fever to be investigated, tuberculosis, typhoid and paratyphoid fever, infective endocarditis, sepsis, intra-abdominal infection or abscess in other parts, biliary tract infection, chronic urinary tract infection, AIDS, and infections caused by viruses, L-type bacteria, spirochetes, rickettsia, chlamydia, and fungi are the main causes; among the connective tissue diseases accounting for 30% of the fever to be investigated, systemic erythroderma is the main cause. Among the connective tissue diseases accounting for 30% of the fevers to be investigated, systemic lupus erythematosus, rheumatoid arthritis, drug fever, subacute thyroiditis, and mixed connective tissue diseases are the main causes; among the neoplastic diseases accounting for 20% of the fevers to be investigated, lymphoma, leukemia, liver and other solid tumors are the main causes. In addition, rare ones include nodular disease, pseudofever, familial Mediterranean fever, and periodic fever. In conclusion, common fevers are sometimes not easy to diagnose, and the process of diagnosis is like solving a case, testing both the doctor’s meticulous thinking and knowledge, as well as the tacit cooperation between doctor and patient. Some patients deliberately do not bring their previous medical histories, trying to “check” whether the two hospitals are in agreement. In fact, the original data and the response to various treatments are important diagnostic or exclusionary clues, and sometimes, a comprehensive analysis of these data may be able to reach a conclusion. Since infection is the first cause of fever to be investigated, prolonged fever of unknown origin may be seen first at an infection clinic. If connective tissue disease is highly suspected, visit the rheumatology clinic, and if hematologic disorders such as lymphoma are highly suspected, visit the hematology clinic.