Crohn's disease is an inflammatory bowel disease (IBD) that affects the digestive system. It can cause inflammation, ulceration, and the narrowing of the intestinal walls. It can also lead to abdominal pain, weight loss, and fatigue. Crohn's disease is an autoimmune disorder, meaning that the body's own immune system mistakenly attacks healthy cells in the digestive tract. The cause of Crohn's disease remains unknown, though researchers believe it is likely caused by a combination of environmental, genetic, and immunological factors. It is thought that bacteria, viruses, or other environmental triggers may interact with a person's genetic makeup to cause the disease. Crohn's disease can affect any part of the digestive system, though it most often affects the ileum, the lower part of the small intestine. Symptoms can include abdominal pain, diarrhoea, rectal bleeding, weight loss, and fatigue. The severity of the symptoms can vary from person to person. Diagnosis of Crohn's disease typically involves a physical exam, laboratory tests, imaging tests such as an X-ray or MRI, and a colonoscopy.