5-year-old baby fever 39.5 ℃ may be caused by pathogenic infections, blood system diseases, connective tissue and other diseases. 1. Pathogen infection: infectious diseases can cause fever, triggered by infections of pathogenic bacteria such as viruses, bacteria, mycoplasma, chlamydia, mycobacteria, etc. These pathogenic microorganisms can cause 5-year-old baby fever 39.5 ℃. 2. Blood system diseases: 5-year-old baby with leukemia, leukemia cells in the patient’s bone marrow cavity proliferation, proliferation at the same time will inhibit the proliferation of normal cells, normal white blood cells, red blood cells, platelets will be correspondingly reduced, their own resistance will be correspondingly reduced, prone to infectious diseases and temperature rise, may have a fever of 39.5 ℃. 3. connective tissue disease: 5-year-old baby with connective tissue disease, such as systemic lupus erythematosus, dry syndrome, dermatomyositis, etc., fever as the first symptom, connective tissue disease essence of autoantibodies and self-antigen formed by the abnormal immune response, the degree of reaction is relatively heavy, the level of inflammation will be higher, which will lead to elevated body temperature, fever 39.5 ℃ phenomenon. 5-year-old baby fever 39.5 ℃ causes are more, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.