There are two most common causes of fever in babies: first, infectious diseases; second, non-infectious diseases. Among infectious diseases, the most common causes are respiratory, digestive, urinary tract, central nervous system and systemic infections. Therefore, the baby fever should also look at whether there are other comorbidities, such as the baby in the fever on the basis of coughing, runny nose, sneezing, nasal congestion, may be respiratory tract infections; if the baby in the fever on the basis of diarrhea, vomiting, stools with mucus, pus, blood, may be the digestive tract infections; if the baby in the fever on the basis of frequent urination, urinary urgency, urinary pain, or even relieve the red urine If the baby has frequent urination, urgent urination, painful urination or even reddish urine on the basis of fever, it may be an infection of the urinary tract; if the baby has poor spirit and convulsions on the basis of fever, it may be an infection of the central nervous system; if the baby has a poor response of the whole body’s mental state on the basis of fever, skin rash, liver and spleen enlargement, it may be a systemic infection. The second major cause of fever in babies is non-infectious, and common diseases include rheumatic diseases, hematologic tumors, and abnormalities of the thermoregulatory center. Therefore, to determine the cause of fever in babies, it is necessary to combine the clinical manifestations of babies to judge.