This refers mainly to 2 different diseases: Crohn Disease and Ulcerative Colitis. They belong to a category of disease called “auto-immune diseases,” when our own immune system attacks the body for reasons that aren’t really understood. In Ulcerative colitis, the rectum is affected; in Crohn, it’s usually the small intestine, but can include any part of the gastrointestinal tract from mouth to anus. Both are more common among persons of Ashkenazi Jewish descent, and among westerners vs. Asians.
Ulcerative Colitis causes diarrhea with blood & mucus. In severe cases, there can be fever, abdominal pain, and weight loss.
Crohn Disease causes a variety of symptoms. Diarrhea may come and go. There may be fatigue, abdominal pain, fever, and weight loss. In the rectum and anus, there may be fissures (cracks) or fistulas (abnormal passages out to the skin, or even internally to the bladder or vagina).
Both Ulcerative Colitis and Chron can inflame a variety of other organs. These include the eye (iritis), joints (arthritis), skin (various painful lumps or sores), lungs (gradual scarring and breathing problems), gallbladder ducts, and more.
Diagnosis is made by biopsy, during colonoscopy (usually). But we need some reason besides just diarrhea to refer patients. They may include any of the symptoms listed above. Also, some simple lab tests like anemia, or an elevated “Sed Rate” or “CRP” may clue us in to an autoimmune disease being likely.
Auto-immune diseases are truly mysterious. There’s treatment available, ranging from simple temporary medications to long-term drugs that slow the immune system, but also put us at risk of various infections; Gastroenterologists know how and when to use these. In advanced cases, it may be necessary to remove part or all of the bowel.
Note that “Inflammatory Bowel Disease” (IBD) is much different from “Irritable Bowel Syndrome” (IBS). IBS never causes the types of complications seen in IBD, nor does it require types of treatment which may carry their own risks. See link for definition of “Syndrome“.