Celiac Disease is an immune system condition that causes the villi in the small intestine(s) to disappear.
This prevents the sufferer from absorbing life sustaining nutrients through the food they eat.
They are gluten sensitive and cannot eat anything with barley, wheat, rye, oats in it. Now think about that! Almost everything you eat has done of those in it!
Even if the food contains none of those things in it, it may be "cross-contaminated" with them through processing.