An ulcer describes any situation where underlying tissue is exposed when it should be protected by overlying tissue. Ulcers can appear on any part of the body, but the best known type is the gastric ulcer. Ulcers can also appear on the skin, in the duodenum, or in the intestines.

Ulcers are always serious conditions that require medical treatment. They are often the result of bacterial infection, but can also be caused by inactivity (bedsores). Whether caused by an infection or not, they provide an opportunity for opportunistic bacteria, such as Staphylococcus sp. or gangrene, to enter the bloodstream. In addition, ulcers can bleed out and cause serious complications from lack of blood.

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