|Cold sores are painful fluid-filled blisters that form on the borders of the lips caused by a herpes virus, most often the herpes simplex 1 virus. Cold sores should not be confused with canker sores, which are small ulcerations in the mouth. The blisters, which are contagious, later break, ooze, and crust over before healing. Recurrences are common and can be triggered by stress, sun exposure, illness, and menstruation.
Cold sores may appear with colds, fevers, exposure to excessive sunlight, or menstrual periods, as well as during periods of stress or illness. The sores usually disappear within two weeks. Initially, there may be tingling or prickling at the site of the cold sores even before they are visible and afterward, the blisters often weep a clear fluid and form a scab. If the infection is transmitted to the eyes, it may lead to blindness.