In addition to the exclusion of physiological factors for low fever, one should be alert to Mycobacterium tuberculosis infection, chronic brucellosis infection, HIV infection, tuberculosis, and various tumors. 1. Mycobacterium tuberculosis infection: Mycobacterium tuberculosis infection will lead to low fever in the afternoon, loss of appetite and night sweating (abnormal sweating after going to sleep, and the sweating stops after waking up). 2. Chronic Brucella infection: after contact with cattle and sheep carrying the germ, there will be persistent low-grade fever, loss of appetite, joint pain and other symptoms. 3. HIV: After HIV infection, there will be chronic diarrhea and chronic cough. There are also some patients with persistent low-grade fever. 4. All kinds of tumors: All kinds of tumors may cause low fever, and the appearance of tumors will lead to tissue necrosis and heat absorption. Commonly, there are lymphoma, which will have persistent low fever, and can be checked by tumor markers if necessary. There are many possibilities for a low fever that doesn’t go away, in addition to all of these diseases, it can also be an immune system disease, a blood system disease, and so on. Therefore, if you have a persistent low-grade fever, it is important to go to the hospital for examination and treatment as prescribed by the doctor.