Diverticulitis develops when bacteria become trapped in pouches (diverticula) that have formed along the wall of the large intestine, leading to an infection. The bacteria grow and cause inflammation and pressure that may lead to a small perforation or tear in the wall of the intestine. Peritonitis, an infection of the lining of the abdominal wall, may develop if infection spills into the abdominal (peritoneal) cavity.

The reason diverticula form in the wall of the large intestine (colon) is not completely understood. Doctors think diverticula form when high pressure inside the colon pushes against weak spots in the colon wall. Uncoordinated movements of the colon can also contribute to the development of diverticula.

Normally, a diet with adequate fiber (also called roughage) produces stool that is bulky and can move easily through the colon. If a diet is low in fiber, the colon must exert more pressure than usual to move small, hard stool. A low-fiber diet also can increase the time stool remains in the bowel, adding to the high pressure. Pouches may form when the high pressure pushes against weak spots in the colon where blood vessels pass through the muscle layer of the bowel wall to supply blood to the inner wall.

It is not known why some people who have these diverticula (a condition called diverticulosis) develop diverticulitis and others do not.