Mouth sores can be caused by infections – bacterial, viral or fungal, a loose orthodontic wire, an ill-fitting denture or a sharp edge from a broken tooth or filling. Some of the most common mouth sores are: *Canker sores, small ulcers with a white or gray base and a red border, *Cold sores are groups of fluid-filled blisters, *Candidiasis, occurs when the yeast Candida albicans reproduce in large numbers. Mouth sores may also be symptoms of an underlying disease or problem. So, if you’ve had any mouth sore that lasts a week or longer, it is a good idea to get your dentist to check it out.