There are many diseases that need to be considered if a patient experiences swelling and pain on one side of the face, including oral inflammation, mumps, local lymph node inflammation, boils or carbuncles on one side of the face, etc. 1. Oral inflammation: such as periodontitis or oral ulcer inflammation increases and spreads to the soft tissues of one side of the face, thus causing inflammation of facial tissues and swelling and pain on one side of the face; 2. Mumps: caused by the mumps virus, which can grow and multiply in the lymphatic tissues and cause local inflammation. The inflammation can spread when it enters the bloodstream and cause swelling and pain when it spreads to one side of the face; 3. Local lymph node inflammation: If the lymph nodes in the lower jaw and upper neck are involved, it may affect the face in severe cases and cause inflammation and swelling and pain in the face; 4. The main clinical manifestations of both are redness, hard nodes, and pain, which can lead to swelling and pain on one side of the face.