Coeliac disease (pronounced see-lee-yak) is a condition causing some adults and children to react to the gluten, the protein found in wheat, barley and rye. Some coeliacs are also sensitive to the protein found in oats. If a coeliac eats gluten the lining of the small intestine becomes damaged reducing the coeliac's ability to absorb the nutrients from food. This can lead to various symptoms and complications if undiagnosed..
It’s an autoimmune disease and it’s not contagious. The only treatment is adhering to a gluten-free diet for life after a positive diagnosis has been made. Medicine or drugs are not required.
In the small intestine there are small finger-like structures called villi which absorb the nutrients from food. When the villi are damaged (or atrophic) they can no longer absorb nutrients properly.

Here are some photographic images of healthy villi compared with damaged villi.
Gluten is in bread, biscuits, cakes, pasta, beer, pizza and in many manufactured foods where gluten is contained in the ingredients such as soups, sauces, gravy, salad dressings, crisps, chocolate, sweets and ready-meals. Coeliacs have to think ahead if they want to eat at a restaurant, need to take extra time reading labels when shopping and need to take care in their personal food preparation to avoid cross-contamination with gluten. Even a grain of gluten can cause harm and cause diarrhoea and other symptoms which can last for several days.