Celiac disease is an autoimmune disease. I know many people think it is simply an allergy, but it is an autoimmune condition, not an allergy. Those will celiac disease must maintain a gluten free lifestyle. Currently, there is no other cure for becoming sick besides avoiding gluten (a protein found in wheat, rye, and barley).
When a celiac person eats gluten the body has an immune reaction that starts attacking the small intestine. These attacks lead to damage on the villi, which are the small fingerlike projections that line the small intestine, and that promote nutrient absorption. When the villi are damaged, nutrients cannot be absorbed properly in the body and can lead to further health problems. Basically, this can cause all kinds of very unpleasant issues for a celiac such as gastrointestinal issues, vomiting, extreme exhaustion, stomach cramps, migraines, brain fog, etc. On top of these short term results, longer, more serious effects can be linked to bowel cancer, infertility in women, anemia, and osteoporosis.
Celiac disease is hereditary. It runs in families. People with a first degree relative with celiac, have a 1 in 10 risk of developing celiac disease.
More scientific information can be found here.
Hopefully you now understand why those with celiac disease need to maintain a gluten free lifestyle!