The exact cause of this disease is not known. However, there are many factors that are thought to be involved with the development of canker sores, including: Weakened immune system; cheese, nuts, and citrus fruits; Stress; Viruses and bacteria; Trauma to the mouth; Poor nutrition and Certain medications. Aphthous stomatitis is usually first seen in children and adolescents from the ages of 10 to 19 years. For about one-third of the children affected, lesions continue to reappear for years after the initial outbreak. These ulcers are not contagious and cannot be spread from one child to another.