Sudden fever in children has many causes, including upper respiratory tract infection, acute gastroenteritis, acute rash, tuberculosis and so on. 1. Upper respiratory tract infections: most common in viral infections, including common cold, influenza, etc., in addition to mycoplasma infections, bacterial infections, etc. can cause fever. 2. Acute gastroenteritis: most of the intestinal flora imbalance caused by unclean diet, acute gastroenteritis can have fever symptoms. In addition, bacterial dysentery is also a cause of fever. 3. Acute rash in young children: the typical symptom is a fever of about 39 ℃, lasting about three days, after the fever goes down, the whole body rash can be seen, which belongs to the self-limiting disease, symptomatic treatment can be. 4. Tuberculosis: it is caused by Mycobacterium tuberculosis infection, which can appear long-term fever, accompanied by lethargy, night sweats, coughing and so on. Sudden fever in children may also have other reasons, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.