Cold Sores are also called fever blisters. Cold sores and fever blisters are caused by herpes simplex virus type 1 (HSV-1). This virus is passed from person to person by saliva (either directly, or by drinking from the same glass or cup) or by skin contact. Cold sores usually appear as clusters of tiny blisters on the lip. Triggers can be a recent illness or fever. Stress also can lead to a cold sore outbreak. This includes mental and emotional stress, as well as dental treatment, illness, trauma to the lips or sun exposure.