Diverticula usually develop when naturally weak places in your colon give way under pressure. This causes marble-sized pouches to protrude through the colon wall. Pouches are most common in your sigmoid and descending colon — the lower portions of your large intestine just above your rectum — and often occur as a result of straining during bowel movements over a number of years. The pouches are small at first but become larger with time.

Sometimes a bit of stool may become lodged in one of the pouches, leading to infection. A small tear or perforation can also develop in an infected pouch, which in turn can cause an infection within your abdomen (peritonitis). If the infection is limited to an area around the wall of your colon where the diverticula are inflamed, you may develop a localized collection of pus known as an abscess.