Mouth sores at the corners of the mouth may be caused by keratitis, but also by herpes on the lips and mouth, and may be caused by impetigo. 1. Keratitis: Keratitis is an inflammatory disease that occurs in the corners of the mouth area at the joint of the upper and lower lips, and may be related to vitamin B2 deficiency. Patients with this disease will have symptoms of mouth sores at the corners of the mouth, and the corners of the patient’s mouth may appear chapped, whitish, swollen, vesicose, and painful. 2. herpes orofacialis: herpes orofacialis is a kind of herpes simplex virus infection caused by the lips and the surrounding skin appeared in a cluster of small blisters of the disease, the patient may appear in the corners of the mouth sores of the performance of the blisters can be broken, scabs, the disease is often secondary to patients with colds or after a fever. 3. Impetigo: Impetigo is a common purulent skin disease, mainly caused by Staphylococcus aureus or type B hemolytic streptococcus infection, the patient can appear in the mouth corners of the symptoms of mouth sores, lesions were herpes, blisters, or pustules, easy to break, with itching. If the patient appears to have mouth sores at the corners of the mouth, should promptly consult a doctor to identify the cause and be disposed of.