Cold sores, also known as fever blisters, are caused by a herpes simplex virus (HSV). There are two kinds of HSV: type 1 and type 2.
If you have cold sores or mouth ulcers, you may have herpes simplex virus 1 (HSV-1). If you have genital herpes, you may have herpes simplex virus 2 (HSV-2)—though type 1 can sometimes spread to the genitals and HSV 2 to the mouth through oral-genital contact.
According to the World Health Organization (WHO), 67% of people under age 50 have HSV-1, but only a minority of them get symptoms like fever blisters. Most people with oral herpes are infected in childhood. Once you're infected, the virus stays in your neurons and can be reactivated throughout your life.