The child’s sudden fever of 39.1 ℃ may be caused by upper respiratory tract infection, acute tonsillitis, acute gastroenteritis, non-infectious fever and other reasons. 1. Upper respiratory tract infection: due to the child’s low resistance and immature development of the respiratory tract, it is more likely to suffer from upper respiratory tract infection. Microorganisms such as viruses or bacteria infecting the upper respiratory tract can cause a child to have a sudden fever of 39.1℃. 2. Acute tonsillitis: Due to the child’s low resistance and immunity, the tonsils are easily infected by bacteria such as staphylococcus and streptococcus pneumoniae. Acute tonsillitis caused by bacterial infection will lead to a sudden fever of 39.1℃. 3. Acute gastroenteritis: children’s gastrointestinal tract function is poor, the resistance to the outside world is poor. Usually eat raw and cold food and unclean food in life will lead to acute gastroenteritis, acute gastroenteritis will lead to sudden fever of 39.1 ℃. 4. non-infectious fever: leukemia, Kawasaki disease and other non-infectious diseases can also cause sudden fever 39.1 ℃. There are more reasons for a child to have a sudden fever of 39.1 ℃, including cystitis, urethritis and other causes, etc. Parents should bring their children to regular medical institutions in time for timely treatment to avoid further progression of the disease.