How do cold sores accur?

Virus. Two kinds of these are common, and both caused by virus. Least worrisome, is the aphthous ulcer. Occurs on the soft tissues of the mouth -- inside lips, cheeks, under tongue. The worse one is herpes.The first time you get it, it can be anywhere in the mouth and outside the mouth. After that , in the mouth, it will only be on tight tissues -- roof of mouth, and right along gum line around teeth.
Herpes virus returns. Cold sores on the lips or near the lips, are usually caused by recurrent oral herpes virus. They start as a slightly raised reddish area, then form a cluster of tiny blisters, which break open soon, leaving a sore (ulcer) to heal up over a week. Sores appear more often when one is under stress or is ill from another illness. Younger babies with herpes should see a doctor when a sore appears.