Cold sores are sometimes called oral herpes because they are caused by the herpes simplex virus type 1 (HSV-1). This virus is very common and highly contagious. It spreads through saliva or close contact — often through kissing or by sharing utensils, straws, towels or lip balm with someone who has a cold sore.
You may not know if you have been infected with HSV-1 because symptoms of exposure to HSV-1 are generally mild. Children sometimes develop a fever and small blisters inside and around their mouths when they are first exposed to HSV-1.