Cold sores are caused by the Herpes Virus. Yes, the same virus that causes them on the privates but a little different strain. Stress often causes outbreaks but is not the cause. So, if you don't use creams or medicine for it, consider taking up meditation or yoga or something that will help you be less stressed.
Cold sores are caused by Herpes Simplex Virus. They are tied in to your immune system. Once activated the host breaks out in lesions around the lips and mouth. Because it is a virus that is triggered by a virus you can't do too much to stop it. Eat healthy, take vitamin C to help fortify your system's defenses.