Cold sores, also known as fever blisters, are small, fluid-filled, painful blisters caused by the Herpes Simplex Virus (HSV). They are caused more often by the HSV-type 1 than the HSV-type 2. These blisters or sores appear on the lips, nose, or around the mouth. The skin around the blisters is sore, swollen, and red. This disease is highly contagious.