Causes. Cold sores are caused by certain strains of the herpes simplex virus (HSV). HSV-1 usually causes cold sores. HSV-2 is usually responsible for genital herpes.However, either type can cause sores in the facial area or on the genitals.hared eating utensils, razors and towels, as well as kissing, may spread HSV-1. Oral sex can spread HSV-1 to the genitals and HSV-2 to the lips. Once you've had an episode of herpes infection, the virus lies dormant in nerve cells in your skin and may emerge as another cold sore at the same place as before.