Although not proven, the dominant theory is that a low-fiber diet is the main cause of diverticular disease. The disease was first noticed in the United States in the early 1900s. At about the same time, processed foods were introduced into the American diet. Many processed foods contain refined, low-fiber flour. Unlike whole-wheat flour, refined flour has no wheat bran. Diverticular disease is common in developed or industrialized countries-particularly the United States, England, and Australia-where low-fiber diets are common. The disease is rare in countries of Asia and Africa, where people eat high-fiber vegetable diets. Fiber is the part of fruits, vegetables, and grains that the body cannot digest. Some fiber dissolves easily in water (soluble fiber). It takes on a soft, jelly-like texture in the intestines. Some fiber passes almost unchanged through the intestines (insoluble fiber). Both kinds of fiber help make stools soft and easy to pass. Fiber also prevents constipation. Constipation makes the muscles strain to move stool that is too hard. It is the main cause of increased pressure in the colon. This excess pressure might cause the weak spots in the colon to bulge out and become diverticula. Diverticulitis occurs when diverticula become infected or inflamed. Doctors are not certain what causes the infection. It may begin when stool or bacteria are caught in the diverticula. An attack of diverticulitis can develop suddenly and without warning.