What is herpes simplex? Most people know it as a cold sore. A cold sore is a common condition we see in patients of all ages. Also known as a fever blister, cold sores are caused by an activation of the Herpes Simplex virus. Often times the initial presentation of an oncoming cold sore is pain or tingling on the lip. This feeling is followed by small groups of red bumps that eventually form into blisters. These blisters may cause discomfort as they begin to dry. After a couple of days a yellow crust may appear that eventually falls off to leave behind fading red areas.