http://www.nlm.nih.gov/medlineplus/ency/article/001150.htm
National Library of Medicine, National Institutes of Health
An inflammation of the lining of the stomach. Gastritis can be caused by infection, irritation, autoimmune disorders, or backflow of bile into the stomach (bile reflux). Infections can be bacterial or viral. Irritation can be caused by medications (such as aspirin or anti-inflammatory drugs), alcohol, chronic vomiting, excess gastric acid secretion (such as from stress), or eating or drinking caustic or corrosive substances (such as poisons). Gastritis can occur suddenly (acute gastritis) or gradually (chronic gastritis).