Diverticulitis is the presence of infected and inflamed diverticula, or small, bulging, abnormal sacs in the large intestine. When a patient has these sacs, the condition is called diverticulosis. The pouches develop mainly in the sigmoid colon, the last section of the large intestine before the rectum. These abnormal sacs usually form due to pressure against weak spots that develop within the colon wall. The diverticula may become infected. The infection can spread if the sac leaks or tears. Abscesses may form outside the colon and other nearby organs (e.g., bladder) may also be affected.

The large intestine absorbs water as it moves fecal matter into the rectum to be excreted from the body. The sigmoid colon, located on the left side of the body, empties into the rectum.

Diverticulitis occurs in people with diverticulosis, a common condition that often develops as people age. Diverticulosis occurs in about half of all Americans over the age of 60, according to the National Institutes of Health (NIH). However, research indicates that acute diverticulitis is becoming increasingly common in younger individuals. Among individuals with diverticulosis, about 10 to 25 percent of patients will develop diverticulitis, according to the American College of Gastroenterology.

Diverticulitis is associated with several complications, including:

• Abscess. If not treated quickly, the infection may escape the diverticulum and form an abscess, which is a collection of pus and infected tissue. An abscess may cause the skin to redden and may be felt during a physical examination as a tender mass in the abdomen.
  
  
• Obstruction. Diverticulitis may cause the intestinal wall to swell and repeated attacks may cause scar tissue to build up and the muscular layer to thicken. This can cause the bowel to narrow (stricture), preventing stool from passing through normally.
  
  
• Fistula. When an abscess forms near another organ (e.g., bladder, small intestine, vagina, skin), it may erode and form an abnormal passageway, or fistula, between that organ and the colon. Fistulae form most commonly between the colon and bladder, often causing severe, persistent urinary tract infections. This occurs far more often in men than in women because the uterus separates the colon and bladder in women.
  
  
• Peritonitis. Sometimes, an abscess from diverticulitis may rupture and the infection may spill into the abdominal cavity, causing peritonitis. This is a very serious complication that may be life-threatening, especially if the contents of the bowel spill out into the abdominal cavity.