Low fever, also known as hypothermia, is a common clinical symptom that generally refers to a body temperature that is above normal but below 38°C. There are various causes of prolonged hypothermia, the most common cause being infection, of which Mycobacterium tuberculosis is the most common, manifested as afternoon hypothermia with malaise, night sweats, and wasting. In this case, you should go to the hospital and have tuberculin test, Tb-spot, sputum smear for Mycobacterium tuberculosis, blood sedimentation, chest X-ray and other related tests. Secondly, functional hypothermia is also common, mainly due to autonomic dysfunction, which affects the normal thermoregulatory process, making heat production greater than heat dissipation and increasing body temperature, including primary hypothermia (more regular fever pattern, smaller temperature fluctuation range), post-cure hypothermia, summer hypothermia (mostly seen in young children, only occurs in summer and mostly occurs in malnourished or brain underdeveloped), physiological hypothermia (mental stress, after strenuous exercise or before menstruation). (after strenuous exercise or before menstruation). In addition, there are some non-infectious causes, such as hyperthyroidism, rheumatic fever, systemic lupus erythematosus, tumors, etc. In addition to routine blood, urine and stool tests, special tests such as blood sedimentation, anti-“O” test, autoantibodies, immunoglobulins, etc. should be done to exclude autoimmune diseases, and tumor markers, CT or MRI tests to exclude tumor diseases. In conclusion, if the prolonged hypothermia is caused by infectious factors, the treatment should be directed at the pathogen of the infection; if it is caused by non-infectious factors, detailed examination should be conducted to clarify the cause and treat the original cause of the hypothermia. Unexplained hypothermia should not be abused with antipyretic drugs.