Cold sores are caused by the herpes simplex virus (HSV). Most cold sores are caused by the type 1 strain (HSV-1). The type 2 HSV strain (HSV-2) usually causes genital herpes but very occasionally can cause cold sores around the mouth. See the separate leaflet called Genital herpes for more details.
This is a companion discussion topic for the original article at https://patient.info/childrens-health/viral-skin-infections-leaflet/cold-sores