Unexplained fever is called unexplained fever and usually has the following three main types of causes: First, inflammatory infectious factors, which usually cause infectious inflammatory diseases such as infections caused by bacteria, viruses, fungi, atypical pathogens such as mycoplasma or chlamydia, when the target organs of the infection such as the lungs, digestive system, urinary system, and reproductive system can cause unexplained fever. Second, non-infectious diseases, which can also lead to unexplained fever, such as immunohistopathies and connective tissue diseases, dermatomyositis, rheumatic fever, rheumatoid arthritis, and systemic lupus erythematosus can also lead to unexplained fever. Third, rare diseases can also produce unexplained fever, for example, tumors, anemia or severe hematologic diseases can present with these symptoms.