Diverticulitis Make an Appointment 413-794-7020 Diverticula are small sacs that form in the lower part of the colon. Diverticulitis develops when these sacs become infected. You may not even know you have these sacs until they become irritated. Diverticulitis is one of the many colorectal conditions the specialists at Baystate Health treat. We’ll work closely with you to determine the best treatment.