If your baby’s fever hasn’t gone down in three days, it may be caused by respiratory infections, gastrointestinal infections, neurological infections, non-infectious diseases and other reasons. 1. Respiratory tract infection: Respiratory tract infection is caused by microorganisms entering the respiratory tract and reproducing in large quantities, mostly due to viral, bacterial, fungal, mycoplasma and other infections, which can manifest in clinical symptoms such as coughing up phlegm, coughing, fever and so on, and a three-day low-grade fever can occur. 2. Gastrointestinal tract infection: Gastrointestinal tract infection is caused by bacterial and viral infections, and the toxins secreted by the bacteria enter into the bloodstream, which will lead to increased body temperature, abdominal pain and diarrhea. 3. Neurological infections: it means that many kinds of pathogenic microorganisms damage the substance of the brain and spinal cord, and in the clinic, there will be convulsions, headache, fever, and weakness, so the baby may have a low fever for three days. 4. Non-infectious diseases: Functional low fever, tumors, trauma, rheumatoid immune diseases and other non-infectious diseases will appear fever. It is recommended to go to the hospital for examination in time and confirm the treatment plan under the guidance of the doctor.