Deep defect in the skin or mucous membrane, formed by the death of tissue. An ulcer shows little tendency to recover, in contrast to a wound which heals much more easily. Skin ulcers occur regularly, and have very varied causes. Bacteria, parasites, fungi etc. can cause ulcers. Examples are leprosy, yaws, leishmaniasis and syphilis. Disorders in the blood supply to a particular part of the skin lead to the death of tissue, with an ulcer resulting (as for example with bedsores). Abnormalities in the nerves can give rise to numbness. As a result, sharp objects are not felt and can easily cause wounds (in diabetes, for example). An ulcer forms a good culture medium for bacteria and gangrene can result. Gastric and duodenal ulcers are regular occurrences. These ulcers arise through the effect of the gastric juices upon the mucous membrane. The resistance of the mucous membrane has usually already been reduced. Tension (stress), operating through the nerves or hormones, may disturb the balance between the resistance of the mucous membrane and the corrosiveness of the gastric acid, with ulcers possibly forming as a result.