Most cold sores are caused by herpes simplex virus type 1 (HSV-1), which usually affects the lips and typically is not transmitted by sexual contact.
Though less common, cold sores may be caused by another type of herpes simplex virus called HSV-2.
Cold sores, caused by a type of virus called herpes simplex type 1, are contagious until they completely go away, which usually takes about two weeks.
Cold sores are the most contagious when fluid seeps out of the sores.