Low fever in 80 year olds is closely related to abnormalities in the body condition. After the thermoregulatory center becomes abnormal, it will cause a mild increase in body temperature, and common causes include chronic inflammation, immune system diseases, tumors, and metabolic diseases. 1. Chronic inflammation: caused by pathogenic infections such as tuberculosis or hepatitis, the pyrogen enters the body and causes low body temperature. 2. Immune system diseases: rheumatism, rheumatoid arthritis, systemic lupus erythematosus and other immune system diseases, because the immune factor can also lead to low fever. 3. Tumor: malignant tumors in various parts of the body, with the progress of the disease, can also lead to low fever. 4. Metabolic diseases: common ones like hyperthyroidism may lead to low-grade fever because of the increased consumption of the body. 80 year olds with recurrent low fever, need to consult a doctor in time to observe, to determine the specific condition and then standardized treatment.