Cold sores are caused by the herpes simplex virus. There are two types of the virus that can cause an infection in or around the mouth or sex organs.
Kissing can also spread the virus. People with herpes simplex can spread the virus even if they have no visible sores or symptoms.
Once people are infected with a herpes virus, they have it for life. Some people will get cold sores again and again, though the sores may be less severe as a person gets older.
The first time children are infected with herpes simplex, they may not feel ill. Some children do get very sick and may have a high fever, swollen glands and painful sores in or around the mouth. They can be sick for a week or more. The sores may hurt so much that the child can’t eat or drink and will need medical care.