Gingivostomatitis (jin-juh-vo-sto-muh-TY-tis) is an infection of the gums and mouth caused by herpes simplex virus type 1 (genital * herpes is caused by herpes simplex virus type 2) and other common childhood viruses. As with herpangina, fever and illness appear before the mouth sores do. The lesions begin as blisters that pop soon after they form, leaving the base of the blister. When this covering peels off, a tender ulcer is formed. It will look grayish or yellowish with a red border. These painful sores often make it difficult to eat.