An oral ulcer (from Latin ulcus) is the name for the appearance of an open sore insie the mouth caused by a break in the mucous membrane or the epithelium on the lips or surrounding the mouth. The types of oral ulcers are diverse, with a multitude of associated causes including: physical or chemical trauma, infection from microorganisms or viruses, medical conditions or medications, cancerous and sometimes nonspecific processes. Once formed, the ulcer may be maintained by inflammation and/or secondary infection. Two common oral ulcer types are aphthous ulcers (canker sores) and cold sores. Cold sores can be caused by the herpes simplex virus.