Diverticulitis is a huge problem for modern, urban dwellers. Fecal matters matter after all and diverticulitis is a prime example. Diverticula are pockets formed in the intestinal wall from the straining and stress associated with constipation and slow-moving intestinal digestion. This is clearly shown by examing third world cultures on a high fiber diet where diverticulitis is virtually unknown. The bottom line is that a fast-moving stool resulting from ample fiber eliminates almost all risk of diverticulitis.
Diverticulitis can be very serious and almost killed UFC star Brock Lesnar. The reason is that these pockets can be so inflamed and infected that they actually ulcerate, i.e. a hole develops, leaking the contents of the intestine into the gut. This is exactly what happened to Brock Lesnar and it dang near killed the hulking MMA star. However, in the great majority of cases, diverticulitis is present with no symptoms or relatively mild symptoms such as diahhrea, constipation or maybe a little rectal bleeding.
What causes diverticulitis and how can you avoid it? The most important thing to do is consumme a lot of fiber.  The Mediterranean Diet and Low Fat Diet that I advocate on this site are prime examples of diets that will help you avoid this modern plague (and boost your erectile strength as well). The reason is that both diets contain ample bulk from whole grains, vegetables and fruit, making for a happy colon.
There are other important causes and risk factors as well, including lack of exercise and being overweight.  Exercise is known for helping move the stool through the intestines. Being overweight increases systemic inflammation and downgrades immunity, all likely contributors to the ills associated with diverticulitis and, indeed, the obese are more likely to have the ulcerative and very dangerous form of diverticulitis.
One study found that it didn't really matter how you defined obese - they all led to increased risk of diverticulitis: "In this large prospective cohort, BMI, waist circumference, and waist-to-hip ratio significantly increased the risks of diverticulitis and diverticular bleeding." 
CAUTION: Tylenol (acetaminophen) and NSAIDs (Ibuprofen, Advil, aspirin, etc.) are associated with the development of diverticulitis. 
The bottom line is that almost every chronic and life-threatening disease that we see in the hospitals and hospices could be avoided through proper diet, exercise and nutrition and diverticulitis is no exception.
1) J of Clin Gastroenterology, 2006, 40(7):S112-S116, "Diverticulitis: New Concepts and New Therapies"
2) Nature Reviews Gastroenterology and Hepatology 6, 1 July 2009, "Large intestine: Exercise decreases risk of diverticular complications"
3) Gastroenterology, 2009, 136(1):115-122, "Obesity Increases the Risks of Diverticulitis and Diverticular Bleeding"
4) Arch Fam Med, 1998 May-Jun, 7(3):255-60, "Use of acetaminophen and nonsteroidal anti-inflammatory drugs: a prospective study and the risk of symptomatic diverticular disease in men"