Cold sores are caused by the herpes simplex virus (HSV). The first time you are infected with this virus is called the primary infection. The mouth is the area most commonly affected. This is because normal skin is resistant to the virus but the moist inner skin of the mouth is not. The first (primary) infection is often in childhood. It is often caught by kisses from a family member who has a cold sore.
This is a companion discussion topic for the original article at https://patient.info/childrens-health/viral-skin-infections-leaflet/primary-cold-sore-infection