A virus called herpes simplex causes them. This name tends to strike fear in the hearts of those who hear it because they recognize this as the virus associated with the sexually transmitted disease genital herpes. However, these viruses, although similar in name, are distinct infections.
Some children acquire the virus and have few, if any, symptoms.
Others get the infection and develop many sores in and around the mouth. These blisters can be quite painful and are associated with a low-grade fever. The children have difficulty swallowing their saliva because of the pain it causes. They may have a poor appetite, and the glands around the neck may swell slightly and be mildly tender. Fortunately, this initial severe infection tends to be a one-time occurrence.