Quick Answers. 1  Herpes stomatitis is a mouth infection caused by the herpes virus. 2  It is most common in children 1-2 years old. 3  Symptoms include many small, sore, open blisters inside the mouth and on the gums and tongue.  Stomatitis is 1  contagious. Stomatitis usually goes away without treatment.  Blisters usually heal in 1-2 weeks.