Gastritis is an inflammation of the lining or mucus membrane of the stomach. This is a phenomenon that can be isolated, due to a bacterium or a virus, but it is often chronic. In cases of chronic gastritis, the cause is mainly due to sustained infection of a bacterium called Helicobacter pylori, or to an autoimmune cause, meaning that it is a side effect of an attack on the mucus membrane by the antibodies in the body. The main symptom is stomach pain, which can be accompanied by nausea, vomiting, and digestive problems. The main complication of gastritis is a gastric ulcer which may lead to gastrointestinal bleeding. The diagnosis of gastritis is made by an endoscopy (a camera inserted through the mouth) and the treatment of the cause is necessary, in addition to treatment of the symptoms.