Diverticulitis is small, bulging sacs or pouches of the inner lining of the intestine (diverticulosis) that become inflamed or infected. Most often, these pouches are in the large intestine (colon). Causes, incidence, and risk factors No one knows exactly what causes the sacs, or pouches of diverticulosis to form. Eating a low-fiber diet is one of the most likely causes. Diverticulosis is very common. It is found in more than half of Americans over age 60. Only a small number of these people will develop diverticulitis. Diverticulitis is caused by small pieces of stool (feces) that become trapped in these pouches, causing infection or inflammation.