Sudden fever in babies may be caused by acute upper respiratory tract infection, measles, infectious mononucleosis and so on. 1. Acute upper respiratory tract infection: usually can be high fever, high fever, cough, sputum, runny and other symptoms. 2. Measles: the typical symptom is also a high fever, which lasts for three to four days, and then the skin of the body develops a rash from the top to the bottom, with the skin intact between the rashes, which can be complicated by pneumonia and encephalitis. 3. infectious mononucleosis: is caused by EBV infection, manifested by high fever, sore throat, generalized lymph node swelling, can be accompanied by skin rash, liver function impairment. There are many other causes of sudden fever in babies, such as toddler emergency rash, pneumonia, etc. It is recommended to consult a doctor in time to identify the cause and cooperate with the doctor’s treatment.