List of what causes mouth ulcer and conditions for which mouth ulcer symptoms are related. If you want to know, "Why do I have mouth ulcer?" consider that mouth ulcer can be caused by a number of illnesses. This is a list of diseases that mouth ulcer is a symptom of, including diseases associated with mouth ulcer. What diseases cause mouth ulcer? Examples of items on this list: Herpes simplex, Syphilis and more. This list of diseases with mouth ulcer as a symptom is in alphabetical order with images when available, though it can also be sorted. If you could say "I am experiencing mouth ulcer," please consult a physician.