Fever pending investigation is defined as a fever lasting more than two weeks, with a peak temperature greater than 38.5°C, which cannot be definitively diagnosed after a detailed history, physical examination and routine laboratory tests. Common causes of fever that cannot be investigated may be cancer fever, autoimmune disease, special pathogen infection, etc. 1. Carcinogenic fever: It refers to non-infectious fever caused by cancer in the process of development. In most cases, there is no special abnormal laboratory test result except for tumor indicators, and the fever symptoms may be mild or severe at times, which may be manifested as flaccid fever, as well as arrested fever or irregular fever, etc. Therefore, it is difficult to be detected. Therefore, it is difficult to be detected. 2. Autoimmune disease: This kind of disease is mainly related to heredity and dysfunction of immune system regulation. When the immune system has an immune response to its own tissues, it can cause fever, which is not easy to be detected by routine clinical examination, and is mostly seen in systemic lupus erythematosus, rheumatoid arthritis and other diseases. 3. Special pathogen infection: such as Mycobacterium tuberculosis infection, which can cause prolonged fever and other symptoms, but routine laboratory tests and anti-infective treatment is ineffective, which can lead to difficulties in finding the cause. Fever can also be caused by other reasons, such as immunodeficiency diseases, etc. For people who cannot find the cause of fever, they need to cooperate with the doctor to conduct a comprehensive examination, to clarify the cause of the disease, and comply with the doctor’s instructions for active treatment.