Crohn's disease. Illustration of Crohn's disease in the terminal ileum (the end of the small intestine). A chronic inflammatory disease, Crohn's can affect any part of the gut with ulcers, which may cause pain, fever, diarrhoea and weight loss. The most common site of disease is the terminal ileum, seen here inflamed and ulcerated where it leads into the large intestine (left). The ileum wall becomes swollen and thickened in patches. A fistula (abnormal connection) has formed between adjacent diseased sections of intestine. The cause of Crohn's disease is unknown. It is treated with corticosteroid drugs, a change of diet, and sometimes surgery.