Lip blisters, medically known as cold sores or fever blisters, are caused by the herpes simplex virus, according to WebMD. There are two types of herpes simplex virus: HSV-1 and HSV2. Both types of this virus can cause cold sores in the mouth area.
The herpes simplex virus usually enters a body because of a break in the skin around or inside a mouth, as stated by WebMD. It usually spreads during kissing, touching an infected individual's saliva, or sharing utensils. Cold sores typically clear up on their own after several days, but skin creams and ointments can be used to speed up the healing process.