Symptoms of a Fever Blister
The first indication a cold sore is developing will be tingling, burning or itching along the lips or inside the mouth. A few days later an oral rash can develop. This rash is followed by vesicles or blister containing a yellow fluid. In some circumstances a fever will accompany a fever blister. The blister will break open and ooze the yellow pus. Eventually the sore dries up and develops a yellow scab.