Celiac disease is an autoimmune disease of the digestive tract. In celiac disease, eating food with gluten damages little bulges in the small intestine. These bulges, called villi, absorb nutrients from foods. The condition affects the absorption of all nutrients. Untreated patients often become malnourished.

Cross Section of Small Intestine
small intestine villi
Inner circle demonstrates little protrusions affected by Celiac disease.
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Doctors do not fully understand what causes celiac disease. Eating gluten seems to be involved. There is most likely a genetic factor. Patients with specific genes develop the disease after exposure to gluten. There is some evidence that earlier exposure in infancy can cause a more severe disease than later exposure.