Celiac disease is a disorder that damages the small intestine and interferes with nutrient absorption. People with celiac disease cannot tolerate gluten, a protein found in wheat, rye, and barley. Celiac disease may cause a wide range of symptoms, such as diarrhea, abdominal pain (or stomach pain), irritability, and depression. Treatment of the disease involves eliminating all gluten from the diet. In order to stay well, people with celiac disease must avoid foods and products containing gluten for their rest of their lives. While research scientists are still trying to determine its exact cause or causes, they believe that celiac disease is a type of autoimmune disease and that certain genes and environmental factors play a role in its development.