It mainly affects the last part of the small intestine. Patches of the gut become inflamed and ulcers may develop but there are sections that are unaffected.
Crohn's can become extremely painful when food and waste build up in areas of the intestine which have become damaged or narrowed by inflammation.
Crohn's disease is an auto-immune condition, where the body produces antibodies that attack the digestive tract. Why this happens is unknown but it's thought to be a combination of genes and environmental factors.
Family history has also been found to increase your risk of developing Crohn's disease, in fact, if you have an identical twin with Crohn's, you have a 70 per cent chance of developing it yourself.
Genes aren't the whole story, one theory is that you are born with a genetic predisposition to Crohn's and its triggered by fighting off a virus or bacterial infection and your body starts attacking its own cells in the process, causing damage to the intestine.
Environmental factors proposed for triggering IBD include smoking, gut bacteria, and the hygiene hypothesis (that we've become too clean and are immune systems have not been challenged enough in childhood).
The most common symptoms are diarrhoea, painful abdominal cramps, fatigue and mouth ulcers plus joint pain. Crohn's can often lead to a loss of appetite, weight loss and anaemia, plus night sweats and it can affect the menstrual cycle.
Over time, the ongoing inflammation can also result in deep ulcers in the bowel wall which sometimes penetrate the layers of muscle, causing scar tissue to form in the bowel.
Crohn's symptoms tend to go into remission and can then flare-up again.
Tests include a general health assessment, stool sample and blood tests.
If IBD is suspected, you will be referred to a specialist called a gastroenterologist. You will then be given a colonoscopy, where a camera is inserted through the back passage (rectum) to investigate the level of inflammation. A tissue sample might also be taken for further investigation.
Who gets it?
Crohn's disease is quite rare, affecting only about one in every 650 people in the UK. You can develop Crohn's disease at any age, but half of people in the UK who are diagnosed with IBD are in their teens or twenties and the average age at diagnosis is under 30 years old.
The main predictor for Crohn's seems to be family history, as one in five people with the disease will have a family member with it too.
Smoking is one lifestyle factor that increases your risk of developing the Crohn's disease and it can also have disastrous effects on those already living with Crohn's as smokers have been found to have a 34 per cent higher relapse rate than non-smokers.
There's no cure for Crohn's disease but treatments can help keep your symptoms in remission and prevent flare-ups as much as possible.
Medication is an important part of the treatment for Crohn's disease, including corticosteroids to reduce inflammation. If you experience two or more flare-ups within a year your doctor may prescribe you immunosuppressant drugs, including azathioprine. Newer treatments include the biologics infliximab and adalimumab, which are immunosuppressants made from natural biological substances including antibodies and enzymes.
Within five years of diagnosis up to 70 per cent of patients with Crohn's disease will undergo some form of surgery.For example, keyhole surgery may be needed to remove a narrowed section of the small intestine that has become blocked (called a stricture).
An ileostomy operation is where the small intestine is diverted through a hole in the stomach (known as a stoma), waste is then collected in a bag. While this can cause some psychological distress, in the long term it can greatly improve a person's quality of life.
The role of diet in IBD is widely debated by doctor and patients but keeping a food diary and monitoring your diet can help to avoid any ‘trigger’ foods.