Cold sores, otherwise known as fever blisters, are caused by a virus called herpes simplex. These infections are contagious and are commonly spread through kissing. Cold sores may develop inside or outside the mouth, as well as on the chin or in the nostrils, and they often come back in the same location. Canker sores are commonly mistaken for cold sores, but they are not contagious.
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American Dental Association answered
Cold sores are very contagious. They are caused by the herpes simplex virus type 1. The initial infection (primary herpes) may be confused with a cold or flu and can cause painful lesions to erupt throughout the mouth. Once a person is infected with primary herpes, the virus stays in the body and causes occasional attacks.
Cold sore blisters usually heal in a week by themselves. Over-the-counter topical anesthetics can provide some relief. Your dentist may prescribe antiviral drugs to reduce these kinds of viral infections.
Cold sores (caused by a common virus called the herpes simplex virus) are very contagious, particularly from the time the blisters appear until they have completely dried and crusted over, so if you have one, avoid close contact with others. If you have any eye irritation along with a cold sore, tell your doctor immediately, as herpes simplex can cause scarring of the cornea and blindness if it spreads to the eye.