Sudden fever without symptoms in children can be caused by the early stages of a cold, gastrointestinal flu, or other systemic infections. It may also be seen as an adverse reaction to vaccination. 1. Early stage of cold: some children will have fever as the main symptom and no other symptoms, but with the progress of the disease, about 2 to 3 days, the child will gradually appear coughing, coughing sputum, sneezing and other symptoms of upper respiratory tract infection. 2. Gastrointestinal colds: Early on, there will be fever, may be accompanied by loss of appetite or digestive symptoms, but the initial stage is mild, followed by nausea, vomiting, diarrhea symptoms. 3. Other systemic infections: such as neurological infections, meningitis or encephalitis patients, the sudden onset of only fever manifestations, and later on, there will be projectile vomiting and so on. 4. Adverse reactions after vaccination: vaccines are inactivated viruses or bacteria, or may be toxoid, may stimulate the thermoregulatory center, resulting in abnormal fluctuations in the thermoregulatory point, which leads to an increase in body temperature. Children suddenly fever without symptoms, there may be other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.