Sudden fever in children is mostly caused by infections, and other factors such as heat stroke, rheumatic immune diseases and hematologic diseases should also be ruled out. 1. Infection: Infection is the most common cause of fever in pediatrics, mostly caused by bacterial, viral, mycoplasma and other infections, the child may have a sudden high fever, accompanied by respiratory or digestive tract and other symptoms. 2. Heat stroke: prolonged exposure to high temperature environment, easy to sudden fever, profuse sweating, dry mouth, etc., the need for timely antipyretic and rehydration and other symptomatic treatment, to avoid heat exhaustion. 3. Rheumatic immune diseases: such as rheumatic fever, sudden onset of fever, accompanied by arthralgia, rash, myocarditis, etc., improve the blood routine, rheumatism, cardiac enzymes, cardiac ultrasound and other clear. 4. Blood system diseases: such as leukemia, reattachment, hemophagocytic syndrome, etc., the child, in addition to fever, often accompanied by sternal pain, anemia, recurrent infections and so on. Children with sudden fever, 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.