Crohn's Disease - Crohn's disease is a disease involving the small bowel most of the time, the colon some of the time, and both the small and large intestines some of the time. The cause of Crohn’s disease is currently not known. The disease involves all the layers of the bowel, and the most common location is where the small bowel and the large bowel join together. Untreated, it usually gradually worsens and often involves more than one region of the bowel with normal sections in between. The most common symptoms of Crohn’s disease are diarrhea, abdominal pain, weight loss, and tenderness in the abdomen. Less common problems can be the development of an abdominal mass, the development of a fistula (an abnormal connection between the bowel and other organs or the skin). In rare instances, Crohn’s disease can also affect the skin, the eyes, the joints and the liver.