Canker sore
A canker sore is a kind of  mouth ulcer. These ulcers occur periodically, and heal completely. They are not contagious.  The medical term for them is Aphthous stomatitis or  recurrent aphthous stomatitis (RAS). The condition is very common. It affects  about 20% of the general population.  It often starts in childhood or adolescence, and lasts for several years, before it slowly disappears. There is no cure, but there are drugs that are mostly given to manage pain.