Coeliac disease is an autoimmune disorder of the small intestine that occurs in genetically predisposed people of all ages from middle infancy onward. Symptoms include chronic diarrhoea, failure to thrive (in children), and fatigue, but these may be absent, and symptoms in other organ systems have been described.
Upon exposure to gliadin, and specifically to three peptides found in prolamins, the enzyme tissue transglutaminase modifies the protein, and the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction. That leads to a truncating of the villi lining the small intestine (called villous atrophy). This interferes with the absorption of nutrients, because the intestinal villi are responsible for absorption.

Very bad!!

Going out to eat can be a pain. Your food bill can skyrocket, even if you are not buying gluten-free replacement items. Because of the gluten-free craze, getting taken seriously can become a bit of a challenge. It can be tough to diagnose (though that may be due more to doctors laziness than the disease itself.)
It can be an ungodly frustrating disease. Even gluten-free, a laundry list of symptoms can come out of nowhere.
People with celiac disease have an increased mortality rate compared to the general population.
It is a life-long disease. Once you are diagnosed, there is no going back to a normal lifestyle.