Cold sores or oral herpes are caused by the Human simplex virus-1 that causes ulcerative lesions of the oral mucosa or lips
. They typically take the form of tiny, clear, fluid-filled blisters on the face, most commonly the lips
. They could become painful blisters that ulcerate and crust over. A cold sore usually clears by itself in seven to twelve days, and rarely leads to medical complications.
The number of blisters varies from one to a whole cluster.