Crohn's disease is an autoimmune disorder of the ileum and/or the intestines. It is caused when the immune system attacks the cells of the intestine, causing inflammation and usually secondary infection in the compromised tissue. Generally, the symptoms are limited to the digestive tract, but in about 15% of patients, symptoms appear in other parts of the body, including the nervous system. Although the disease is rarely fatal, its complications can lead to serious infection or even stroke.
Although Crohn's isn't terribly rare (It occurs in about one out of every 15,000-20,000 people), it is often classified as a zebra diagnosis as the gastrointestinal symptoms are common in many diseases and the more serious complications are not likely to be caused by Crohn's, but by more common diseases. There is no definitive diagnostic test for the disease, and the physician must diagnose Crohn's by ruling out conditions that present in a similar manner. For example, Crohn's is often mistaken for ulcerative colitis, but cannot be treated in the same manner. However, it is often impossible for a physician to distinguish the two even if they have both illnesses in the differential.
Crohn's is not curable, but it is treatable. In most cases, the symptoms won't disappear without treatment, but without treatment, they will almost certainly return. For acute cases, the recommended treatment is antibiotics to deal with the infections that often arise, plus steroids to reduce the inflammation. In the longer term, dietary changes seem to be effective - smaller meals at more frequent intervals, plus the avoidance of foods (such as high fiber foods) that bring on symptoms. Proper hydration also seems to be effective in preventing attacks. Smoking appears to bring on attacks, so sufferers are instructed to quit.