Fever is one of the most common reasons for visits to pediatric clinics. In general, fever is considered to be present when the anal temperature exceeds 37.8°C, the sublingual temperature exceeds 37.5°C, and the axillary temperature exceeds 37.4°C. Fever in newborns and infants less than 3 months of age is defined as an anal temperature greater than or equal to 38°C. Fever is only a symptom, and the diagnosis of fever can be made by taking the temperature, but the diagnosis of the cause of fever is not easy, especially in small infants, who may have a serious infection, and the fever and accompanying symptoms should be carefully observed. Infection is the most common cause of fever in small infants. Infants with atypical fever symptoms need to be carefully observed for concomitant symptoms such as the presence of cough, runny nose, vomiting, diarrhea, milk consumption and urination and defecation, and the presence of toxic manifestations: including features such as irritability, difficulty in calming, poor perfusion (pale, grayish or cold and wet flushed limbs), hypotonia, reduced activity or lethargy; and characteristic manifestations of local infection: umbilicus, skin lesions, arthritis, swollen limbs, etc. If there is poor mental health, toxic manifestations, etc. suggesting severe infection, viruses causing severe infection include herpes simplex virus, chickenpox, enterovirus, influenza virus, certain adenoviruses and respiratory syncytial virus. Severe bacterial infections usually arise from urinary tract infections, bacteremia, bacterial gastroenteritis, bacterial meningitis, and skin or soft tissue infections. Fever can be the only sign of a potentially serious infection in a small infant, and parents see only the surface. Prompt medical attention is needed to assess the severity of the condition, which may require routine blood and urine stool tests, and even chest radiographs, blood cultures and cerebrospinal fluid tests. It is important to seek prompt medical attention for fever in young infants, so that treatment is not compromised by delays.