Diverticulitis. X-ray of the colon of a patient with diverticulitis. A diverticulum is a small pouch-like structure that protrudes out of weak spots in the intestinal wall. Diverticula are thought to be caused by a low-fibre diet and lack of exercise. In most cases diverticula are symptomless and cause no problems. Occasionally they can become inflamed and infected, a condition called diverticulitis, causing abdominal pain and constipation.