Usually persistent high fever is caused by the following factors: 1. The most common factor is infectious, such as persistent high fever when the body is infected by bacteria, viruses, fungi and other atypical pathogens, such as mycoplasma or chlamydia. 2. Persistent high fever can also occur in some immune diseases, connective tissue diseases and rheumatic diseases, such as rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis and dry syndrome, 3. In some cases of immune system dysfunction or extremely low immune status, such as severe tumors, severe anemia, blood system diseases, leukemia, AIDS, etc., the above symptoms may also occur.