Fever in the elderly is mostly caused by infectious diseases, tumor diseases, autoimmune diseases and so on. 1. Infectious diseases: including bacterial, viral, fungal, parasitic and other biological pathogens infections, which can cause tuberculosis, urethritis, ascariasis, septicemia and so on, which are harmful to human health and lead to the increase of body temperature. 2. Tumor diseases: including liver tumor, lung tumor, pancreatic tumor, stomach tumor, cervical tumor, ovarian tumor, lymphoma, leukemia, etc. Fever caused by tumor diseases may be infectious fever or fever caused by tumor itself, which is also called tumor fever. 3. Autoimmune diseases: including lupus erythematosus, rheumatoid arthritis, ankylosing spondylitis, etc., which can also lead to fever in human body. Patients may have multiple organ, structural and functional damage throughout the body, leading to weakness, fatigue and fever. Recurrent fever in the elderly can be caused by a variety of other conditions. It is recommended to go to the hospital for examination to find the cause of the fever and to treat the primary disease.