An 80 year old man who has fever every night may be suffering from the presence of bacterial viral infections such as pneumonia, tuberculosis infections, urinary tract infections, influenza, etc., as well as the presence of tumors, connective tissue diseases, etc. To identify the cause, it is necessary to go further to the hospital to improve laboratory tests. The most common cause of fever is caused by bacterial or viral infections. Elderly people with lower immunity are prone to bacterial and viral infections that cause illnesses and fever, such as influenza, pneumonia, tuberculosis, acute gastroenteritis, acute urinary tract infections and so on. In addition to infectious causes of fever, there may be tumors or immune system-induced non-infectious fever, commonly seen in some connective tissue diseases such as rheumatoid arthritis, ankylosing spondylitis, vasculitis. To clarify the cause, it is recommended to take the elderly to the hospital to improve blood tests, related CT and other tests to clarify the diagnosis.