There are many causes of hypothermia, including physiologic and pathologic ones. Physiologic hypothermia usually occurs around the time of a woman’s menstrual period. In addition, low-grade fever may also occur after overwork or emotional stress. On the other hand, pathologic factors include infections, tumors and immune system diseases: 1. Infection: when bacteria and viruses invade the body and cause infection, it may cause low fever, such as pneumonia, tuberculosis, bone and joint infections, central system infections and so on. 2. Tumor: some solid tumors or tumors of blood system can cause unexplained low fever. 3. Immune system diseases: such as rheumatoid, systemic lupus erythematosus and so on. After the occurrence of unexplained low-grade fever, it is recommended to go to the hospital immediately for relevant examination to find the cause of the disease and actively receive treatment.