Low fever is common in infectious diseases, such as respiratory infections, to which we are more exposed. Patients with Mycoplasma pneumoniae and Chlamydia infections also have prolonged low-grade fever and are not well treated with cephalosporins or penicillins. In addition, hypothermia is also commonly associated with some rheumatic immune diseases, malignant tumors, blood diseases and some infectious diseases. There are many causes of prolonged hypothermia, besides Mycoplasma pneumoniae and Chlamydia infection, followed by Mycobacterium tuberculosis infection, such as common pulmonary tuberculosis, manifested as cough, coughing sputum, blood visible in sputum, low fever in the afternoon, accompanied by weakness and night sweats, etc. Positive sputum Mycobacterium tuberculosis culture can make a clear diagnosis. Some functional hypothermia is mainly due to autonomic dysfunction, resulting in dysfunction of normal thermoregulation, which makes heat production greater than heat dissipation and increases body temperature. If the patient has a clear history of unclean sexual life, it is necessary to consider infection with AIDS, or syphilis or gonorrhea, in which case the patient will have a prolonged hypothermia. There are also non-infectious causes such as hyperthyroidism, malignant tumors, rheumatic fever, and menstrual cramps during the physiological period, which may also cause hypothermia. Patients with low fever should remember not to use antipyretic drugs blindly until the cause is identified. Persistent low fever should be promptly seen by a hospital. Relevant hematological and imaging examinations should be completed to identify the cause before the right treatment can be given.