Oral Herpes (Cold Sore) Cold sores are small, painful blisters that usually appear on or around the lips. They are also called fever blisters or oral herpes. Cold sores are most often caused by the herpes simplex virus, commonly HSV-1. Once someone has the virus, it can stay in the body and reactivate later, causing repeat outbreaks. Cold sores often develop on the lip or around the mouth and are caused by HSV. Cold sores are common, but they can be uncomfortable, embarrassin