5 months baby fever 38 ℃ may be vaccinated, infectious fever, autonomic dysfunction and other causes. 1. vaccination: vaccines are viruses, bacteria and other pathogenic microorganisms and their metabolites, artificial attenuation, inactivation and other methods made of immune preparations, into the human body to activate the body’s immune system, causing an immune response, the body has a mild to moderate fever, the body temperature is often around 38 ℃. 2. Infectious fever: 5-month-old baby due to viruses, bacteria and other pathogenic bacteria infection triggered by the body infection, pathogenic bacteria into the blood circulation as a pyrogen, causing endogenous pyrogen generation, the role of the thermoregulatory center caused by fever, body temperature may be around 38 ℃. 3. autonomic nervous system dysfunction: 5 months baby long time lack of vitamin D, may cause autonomic nervous system dysfunction, have night sleep disturbance, night terrors and other symptoms, may cause recurrent fever, usually low fever, body temperature does not exceed 38 ℃. 5 months baby fever 38 ℃, there may also be other reasons, 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.