Sudden fever in one-year-old babies may be due to physiological factors, infections, gastrointestinal disorders, rheumatic immune diseases, oncological diseases and so on. 1. Physiological factors: such as strenuous exercise, crying, wearing too thick clothing, room temperature is too high, emotional excitement, etc., can cause temporary fever. 2. Infection: for example, the upper respiratory tract is infected by virus, bacteria, mycoplasma, parasites and so on, and the lungs are infected by Mycobacterium tuberculosis, all of which can cause symptoms of fever. 3. Gastrointestinal dysfunction: If the baby suddenly eats more, indigestion occurs, or eat unclean food caused by intestinal flora imbalance, etc., fever may occur. 4. Rheumatic immune diseases: such as rheumatic fever, systemic lupus erythematosus, etc., can appear fever. 5. Neoplastic diseases: leukemia is one of the most common malignant tumors in children, which can also be manifested by fever. More than a year old baby suddenly 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.