What are cold sores?

Type 1 herpes simplex virus.
    Herpes simplex virus type 1 ("HSV1") generally only infects those body tissues that lie "above the waistline" and it is HSV1 that causes cold sores in the majority of cases.
Type 2 herpes simplex virus.
    Herpes simplex virus type 2 ("HSV2") usually only infects those body tissues that lie "below the waistline" and it is this virus that is also known as "genital herpes." Herpes simplex virus type 2 is not usually the virus that causes cold sores, although it can. When you think of having an infection you usually assume that you were recently exposed to the germ that has caused your problem. With cold sores this is not the case. Cold sores are not a sign of a recently acquired herpes simplex infection but instead a reactivation of herpes virus particles already living in your body. Cold sores occur when latent herpes simplex virus particles, which have been lying dormant ("asleep"), become reactivated. The dormant virus particles come from a previous herpes infection. A person's initial herpes simplex virus infection, termed "primary herpetic stomatitis," does not usually take the form of a cold sore and therefore a person may not relate their initial exposure to the herpes virus to the recurrent cold sores that they get. If you don't remember having had the classic signs and symptoms of primary herpetic stomatitis it might be because you experienced them as a small child. Most cases occur before the age of 7.

In other cases, if you don't remember having all the classic signs and symptoms, it's probably because your case was subclinical. This is by far the most common occurrence. It has been estimated that 99% of all cases of primary herpetic stomatitis are subclinical, meaning that the course the infection ran was so slight that its symptoms were not apparent. It is quite possible that when you had the infection that, at most, you only experienced one or two mild mouth sores.