This info is freely available on the 'net. From the U.S. National Library of Medicine:
While the exact cause of Crohn's disease is unknown, the condition is linked to a problem with the body's immune system response.
Normally, the immune system helps protect the body, but with Crohn's disease the immune system can't tell the difference between normal body tissue and foreign substances. The result is an overactive immune response that leads to chronic inflammation. This is called an autoimmune disorder.
People with Crohn's disease have ongoing (chronic) inflammation of the gastrointestinal tract. Crohn's disease may occur in any area of the digestive tract. There can be healthy patches of tissue between diseased areas. The inflammation causes the intestinal wall to become thick.
There are different types of Crohn's disease, depending on the part of the gastrointestinal tract that is affected. Crohn's disease may involve the small intestine, the large intestine, the rectum, or the mouth.
A person's genes and environmental factors seem to play a role in the development of Crohn's disease. The body may be overreacting to normal bacteria in the intestines.
The disease may occur at any age, but it usually occurs in people between ages 15 - 35. Risk factors include:
Family history of Crohn's disease
Symptoms depend on what part of the gastrointestinal tract is affected. Symptoms range from mild to severe, and can come and go with periods of flare-ups.
The main symptoms of Crohn's disease are:
Crampy abdominal (belly area) pain
Loss of appetite
Pain with passing stool (tenesmus)
Persistent, watery diarrhea
Unintentional weight loss
Other symptoms may include:
Fistulas (usually around the rectal area, may cause draining of pus, mucus, or stools)
Rectal bleeding and bloody stools
Skin lumps or sores (ulcers)