Cold sores are caused by a contagious virus called herpes simplex. There are two types of herpes simplex virus. Type 1 usually causes oral herpes, or cold sores. Type 1 herpes virus infects more than half of the U.S. population by the time they reach their 20s. Type 2 usually affects the genital area (source: National Institute of Health, http://www.nlm.nih.gov/medlineplus/coldsores.html).
However, even if genital herpes are caused by Herpes Simplex Virus type 2 (HSV-2), a new study shows some cases of genital herpes that result to HSV-1 according to Rhoda Ashley-Morrow a doctorate researcher at the University of Washington. The study also shows that in the United Kingdom there are as many as 60% of new genital herpes infections that are transmitted due to HSV-1 and in Seattle, USA a third of new cases is caused by the same type of herpes simplex virus.