Last a week. Cold sores (also called fever blisters.) groups of fluid-filled blisters that often erupt around the lips and sometimes under the nose or around the chin. Cold sores are caused by the herpes simplex virus type 1 and are very contagious. 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 wi.