text: There are a number of things that could cause a mouth sore such as bacteria, virus, fungal infection, tooth abcess, sharp edge from a a broken tooth or filling could be some of the culprit. If the sore has lasted more than a week after your dentist check it. Meanwhile avoid things that irritate it like acidic food (orange juice, lemon juice).also check out American Dental Association link below for photos of some of the more common mouth sores.\nhttp://www.ada.org/public/topics/mouth_sores.asp\nhttp://www.nwsmilespecialist.com