Cold sores are clusters of small, painful, fluid-filled blisters, normally reddish in color and containing a white centre, that can appear one at a time or in little bunch. They are usually found on the lip (typically occurring on the margin of the lip where the red mucosa meets skin), the outer edge of the mouth or sometimes, the bottom of the nose, gums or roof of the mouth.

Cold sores, also called fever blisters, are quite different from canker sores, a condition people occasionally associate them with. Canker sores are small sores that occur inside the mouth and you can get them on the tongue and on the inside of the cheeks and lips — basically the parts of the mouth that can move. The big difference though between canker sores and cold sores is that canker sores are not contagious whereas cold sores are very contagious and remain that way at the very least until the blisters scab over. The sores usually crust over within 4 days and heal completely within 8 to 10 days.

An outbreak of cold sores occurs when latent herpes simplex virus particles, which have been lying dormant in your body, become reactivated. With this in mind, an outbreak is not necessarily a sign of a recently acquired herpes simplex infection and not everyone who gets the herpes simplex virus develops cold sores.

Cold sores are generally not dangerous and do not cause any permanent damage to the skin. Some sufferers who have recurring cold sores claim to feel a tingling or slight itching of their lip 1 or 2 days before the blister appears.