according to web md - Aphthous ulcer (pron.: /??f??s/ af-th?s), also known as a canker sore, is a type of mouth ulcer that presents as a painful open sore inside the mouth or upper throat characterized by a break in the mucous membrane. Its cause is unknown, but they are not contagious. The condition is also known as aphthous stomatitis (stomatitis is inflammation of the mucous lining), and alternatively as Sutton's Disease, especially in the case of major, multiple, or recurring ulcers.