Herpes simplex virus
Cold sores are small blisters that develop on the lips or around the mouth. They're caused by the herpes simplex virus and usually clear up without treatment within 7 to 10 days
You won't usually have any symptoms when you first develop the herpes simplex virus (a primary infection). An outbreak of cold sores may occur some time later.
Cold sores are usually caused by the herpes simplex virus type 1 (HSV-1). Occasionally, they can also be caused by the herpes simplex virus type 2 (HSV-2).
Cold sores usually clear up without treatment within 7 to 10 days. Antiviral creams can be used to ease your symptoms and speed up the healing time.