A disease in which the immune system mistakenly attacks the body's own cells and tissues. Sometimes the whole body is attacked, and sometimes only one organ (as in celiac disease, in which only the small intestine is attacked).
The reason(s) why the immune system dysfunctions in this manner are not well understood but are probably related to a combination of genetic and environmental factors. Bacteria, viruses, and toxins can sometimes trigger an autoimmune disease in someone who carries a genetic predisposition for it.
Celiac disease is just one of many autoimmune disorders. (Others include type 1 diabetes, autoimmune hepatitis, Graves disease and Hashimoto's thyroiditis, rheumatoid arthritis, systemic lupus erythematosus, scleroderma, to name just a few.)