Ulcerative colitis is a chronic inflammatory bowel disease in which the lining of the rectum and colon becomes inflamed and develops ulcers. Symptoms include bloody diarrhoea, abdominal pain, urgency, fatigue, and weight loss, with flare-ups alternating with remissions. It is an autoimmune condition — the immune system attacks the gut lining — and its cause is unknown, though genetics, gut microbes, and environment all contribute. There is no cure short of removing the colon, but medicines from anti-inflammatories to biologics control most cases. Diet does not cause ulcerative colitis, but during flares patients often need low-residue, easily digested foods, and nutritional deficiencies from chronic blood and nutrient loss must be corrected.