The causes of low-grade fever at night can be divided into three major categories as follows: 1. Infectious fever, such as bacterial infections, viral infections, fungal infections and atypical pathogens, such as mycoplasma, chlamydial infections, mycobacterium tuberculosis, etc. The body is attacked by various immune microorganisms during this infection, and the antigenic antibody response in the body is provoked to expel the pathogenic microorganisms. episodes of low-grade fever occur when the pathogenic microorganisms are expelled from the body, and the fever is especially significant at night, commonly seen in pyelonephritis, gastroenteritis, peritonitis, etc. 2. some non-infectious inflammatory reactions, commonly seen in connective tissue diseases, immune diseases, systemic lupus erythematosus, rheumatism, rheumatoid disease, dermatomyositis, etc., also present with low fever as soon as evening comes. 3. Endocrine diseases, such as hyperthyroidism and hyperadrenocorticism, also tend to present with low fever as soon as evening comes.