The tingling sensation is caused by the herpes simplex virus waking from its dormant state and making its way to the surface of the skin from where it usually resides. The virus replicates and destroys skin cells at the surface, which causes then blister, which in fact is made up of many tiny blisters that merge.
Treatment of cold sore symptoms is best started as soon as the tingling sensation is felt, and should not be left more than 24 hours if it is to be at all effective.
