Herpes simplex gingivostomatitis is inflammation of the gums and lips caused by the herpes virus; the virus that causes cold sores. Infections in children often go unnoticed but approximately one child in four will develop mouth blisters after their first infection. Your child may have a fever and become irritable, then a day or two later develop ulcers on the lips and gums. These are painful and often lead to drooling and a refusal to eat or drink. Your child may then become dehydrated. The blisters usually take from 10 -14 days to go away. The blisters never leave scars. The infection can be passed on to other children if they come into contact with fluid from the blisters.