There are many reasons that can cause fever in the elderly, which may be caused by external environmental factors, infectious diseases, immune diseases, tumors and other pathological factors. 1. External environmental factors: If the elderly are in a hot environment, their body temperature will rise due to the heat of the external environment. This situation is usually temporary, after improving the temperature of the environment, the fever of the elderly will be relieved. 2. Infectious diseases: Infection is a common cause of fever, the elderly due to decreased immunity, more likely to be infected by external pathogens causing fever. For example, urinary tract infection, respiratory tract infection, acute gastroenteritis, pneumonia and so on. 3. Immune diseases: If the immune system of the elderly has abnormalities, resulting in immune diseases, can also cause fever, such as systemic lupus erythematosus, rheumatoid arthritis and other diseases. 4. Tumor: if the old man suffers from tumor, especially malignant tumor, such as leukemia. There are many reasons for fever in the elderly, if it is not caused by external environmental factors, it is recommended to seek timely medical treatment to avoid delay.