Celiac Disease is an autoimmune disease where someone is allergic to wheat, rye, oats, and barley, proteins found in gluten. Although oats are debatable…in Canada they are considered gluten free- however in the US they are not (I personally get sick if I eat them). According to Celiac.com the prevalence of Celiac Disease is now 1 in every 133 people in the US.
When someone with Celiac Disease eats any bit of gluten, they become extremely sick showing symptoms, although not everyone will show symptoms or will only show symptoms much later on in life. Someone with this disease will not be able to process gluten properly. This will cause the small intestine (which absorbs food) to become damaged, causing the villi (finger-like projections that grad and absorb your food) will lie flat, which in turn causes your body to not be able to absorb proper nutrients. Literally everything that they will eat, will only pass right through them, no matter what they eat, they will not absorb any of the nutrients since the villi lie flat.
The younger you are when you get diagnosed, the better your chances are of making a faster recovery (since your body is young it can itself repair quicker than someone who is older) and to avoid other side effects/other autoimmune disorders in the long run.
Be weary of doctors who diagnose too quickly. Make sure and get tested if you suspect this is what you have. Although it was about six years ago, when I was going in and out of doctors, all they wanted to do was ask if I was Anorexic or pass me along to another doctor. In the end it ended up being my primary physician who ordered the tests be done to check for possible gluten intolerance.
*Statistics taken from Celiac.com