Diverticulitis is caused by the infection and resulting inflammation of diverticula, or small abnormal sacs, in the large intestine. The diverticula may become infected when stool is caught in them. This may interrupt the blood flow to the diverticulum and allow bacteria to accumulate. Infection may also occur when the wall of a diverticulum erodes and forms a small tear. It was once believed that diverticulitis was caused when stool became trappedin the diverticula, but this is no longer believed to be true.

The most prominent risk factor for diverticulitis is diverticulosis (a condition characterized by the presence of diverticula). However, according to the American College of Gastroenterology (ACG), only 10 to 25 percent of patients with diverticulosis will ever develop diverticulitis. Other risk factors include eating a high-fat, low-fiber diet, inadequate exercise and frequent constipation.

The risk of developing both diverticulosis and diverticulitis increase with age. The body becomes less efficient at waste removal and the elasticity and strength of the colon decrease with age, especially in the sigmoid colon. However, diverticulitis may be more severe and may pose a greater chance of complications in patients under the age of 40 years.