You can't see them and they pose no real risk to your health, but they kill. If you've ever suffered from canker sores (those small, round ulcers that develop on the lining of the cheeks and lips, the tongue, or the base of the gums), you know how they can make the simplest things—eating, drinking, brushing your teeth—a real pain. "Canker sores are caused by an opportunistic virus that is always present in the mouth," explains Matthew Messina, D.D.S., consumer advisor for the American Dental Association. "The virus attacks when your resistance is down [due to stress…