Celiac disease is an inherited condition that damages the small intestine and interferes with nutrient absorption. People who have it cannot tolerate a protein called gluten, which is found in wheat, rye, and barley. The disease can cause a wide range of symptoms, such as diarrhea, abdominal pain, irritability, and depression. Celiac disease is treated by eliminating all gluten from the diet. In fact, people with the disorder must follow a gluten-free diet for the rest of their lives.