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.