Cold sores or fever blisters are a blister or group of blisters that appear on the lip or around the mouth. According to WebMD the cause of cold sores is herpes simplex virus, once herpes simplex virus is in your body it is there to stay . Cold sores are often confused with canker sores which are not contagious and are distinguished by a small, round white circle haloed by a sharp halo of redness.