Fever blisters are one of the most common conditions of the mouth. Also known as cold sores (and known medically as herpes labialis), fever blisters are caused by a contagious virus called the herpes simplex virus.There are two types of herpes simplex viruses: 1  Herpes simplex type 1 (HSV-1).n fact, more than 95 percent of recurrent fever blister outbreaks are caused by HSV-1. Most people infected with the HSV-1 virus usually become infected before they are 10 years old. On the other hand, HSV-2 most commonly causes genital herpes (see Genital Herpes Causes) .