Herpes simplex is the most common virus that infects humans. Ninety percent of people worldwide have this virus at sometime in their lives. It is passed easily from one person to another by direct contact with a herpes blister, and once in the body, never leaves. It stays in the nerve cells, and may lie inactive for several years before causing another sore. Cold sores can be brought on by sun or windburn, by stress, or as a result of an illness that weakens the body's immune system.
Cold sores are also more likely to happen in women during their monthly menstrual periods.
Signs that a cold sore is about to form include a tingling or burning sensation at the place where the cold sore is about to develop, accompanied by itching. Within a couple of hours, the sore is noticeable. It appears first as small, red bumps that fill with watery fluid. The bumps may join together to form one large blister. The blister is painful and itchy.