Diverticulitis happens when small pouches that form in the wall of your colon become inflamed or infected, typically through a combination of trapped stool, bacterial overgrowth, and tiny tears in the pouch lining. About 60% of people over 60 have these pouches (called diverticula), and most never know it. But when one of those pouches gets blocked or irritated, the result can range from mild pain to a serious infection.
Understanding how this process unfolds, step by step, helps make sense of why certain people get it and what you can do to lower your risk.
How the Pouches Form in the First Place
Your colon wall has layers of muscle that contract to move digested food along. Small blood vessels called vasa recta pass through the muscular layer to supply the colon’s inner lining. The spots where these blood vessels penetrate create natural weak points in the muscle, like tiny gaps in armor. Over time, pressure inside the colon can push the inner lining outward through those weak spots, forming small balloon-like pouches.
These pouches are technically “false” diverticula. That means the full muscular wall doesn’t bulge out. Instead, just the inner lining and the tissue beneath it herniate through the muscle, covered on the outside by only a thin membrane. This makes the pouches structurally fragile, with thinner walls than the surrounding colon. Having these pouches is called diverticulosis, and by itself, it usually causes no symptoms at all.
What Turns a Pouch Into a Problem
Diverticulitis occurs when one or more of those thin-walled pouches develops a micro- or macro-perforation, allowing bacteria that normally stay inside the colon to cross the mucosal barrier. The traditional explanation is straightforward: a small piece of stool gets lodged in a pouch, blocking its opening. That blockage leads to stagnant material inside the pouch, irritation of its lining, and restricted blood flow. The tissue becomes damaged, bacteria multiply, and inflammation sets in.
The more current understanding is that the process involves a complex interaction between your diet, your immune system, and the balance of bacteria in your gut. When the bacterial ecosystem in your colon shifts in certain ways, the protective barrier lining your intestine weakens. Bacteria that wouldn’t normally cause trouble can then invade the thin wall of a diverticulum, triggering an inflammatory cascade. In mild cases, this means localized swelling and pain. In severe cases, the pouch can rupture, leading to abscess formation or even widespread infection in the abdominal cavity.
The Role of Gut Bacteria
Your colon contains trillions of bacteria, and the specific mix matters. In people with diverticulitis, researchers consistently find a reduction in bacteria that produce short-chain fatty acids. These fatty acids act as fuel for the cells lining your colon and help maintain a strong intestinal barrier. When the bacteria that produce them decline, the barrier weakens.
At the same time, potentially harmful bacteria from the Enterobacteriaceae family tend to be overrepresented in people with active diverticulitis. Anti-inflammatory bacterial groups, including certain Lactobacilli and Bacteroides species, are often depleted. The net effect is a gut environment that’s less able to keep inflammation in check and more vulnerable to infection when a pouch is stressed. Researchers have also found that the bacterial composition on inflamed tissue differs from the bacteria on healthy tissue just centimeters away, suggesting these microbial shifts are closely tied to the local disease process rather than being a whole-gut phenomenon.
Genetics Account for Nearly Half Your Risk
Twin studies estimate that 40 to 53% of an individual’s susceptibility to diverticular disease comes from inherited genetic factors. That’s a surprisingly large contribution, placing diverticulitis in the same category as many common chronic diseases where genetics and environment share roughly equal influence. If a close family member has had diverticulitis, your own risk is meaningfully higher, independent of your diet or lifestyle.
The remaining risk comes from modifiable factors, which means you still have significant leverage over whether those inherited vulnerabilities ever become a problem.
Diet, Weight, and Lifestyle Factors
Low fiber intake has long been considered a primary driver, and the data supports that connection. One large study found that people consuming around 34 grams of fiber per day had a 23% lower risk of developing diverticulitis compared to those eating about 15 grams daily. Most adults in Western countries fall well short of 34 grams. Fiber adds bulk to stool and helps it move through the colon more efficiently, which reduces the pressure that pushes pouches outward and decreases the chance of stool getting trapped inside them.
Obesity is another major factor, and its influence has grown dramatically. As obesity rates in the United States climbed from 12% in 1970 to 49% in 2007, the proportion of patients diagnosed with diverticulitis rose from 19% to 40% over roughly the same period. Excess body weight increases chronic low-grade inflammation throughout the body and raises intra-abdominal pressure, both of which stress the colon wall.
Why Younger People Are Getting It More Often
Diverticulitis was once considered a disease of aging, but that picture has changed. Between 1998 and 2005, hospital admissions for diverticulitis in patients under 45 increased by more than 70%. In some recent studies, patients under 50 account for 18 to 34% of acute diverticulitis admissions, up from just 2 to 7% in earlier decades. One study of 238 patients found that 26% were under 40, with an average age of 32.
The likely explanation is a combination of rising obesity rates, lower dietary fiber intake, and more sedentary lifestyles among younger adults. Greater use of CT scanning may also play a role in catching cases that would have gone undiagnosed in earlier eras.
Medications That Increase Risk
Common over-the-counter pain relievers can raise your chances of developing diverticulitis or make an existing case worse. Regular use of non-aspirin anti-inflammatory drugs (like ibuprofen or naproxen) is associated with a 72% increased risk of diverticulitis compared to nonuse. For complicated cases involving perforation or abscess, the risk jumps even higher, to about 2.5 times that of nonusers.
Regular aspirin use carries a smaller but still meaningful 25% increase in diverticulitis risk. Combining aspirin with other anti-inflammatory drugs raises the risk further, and roughly doubles the risk of diverticular bleeding. These medications work by suppressing inflammation body-wide, but they also weaken the protective lining of the gut, making the already-thin walls of diverticula more vulnerable to perforation.
What Happens When It Becomes Severe
Most diverticulitis episodes are uncomplicated, meaning the inflammation stays localized near the affected pouch. You’ll typically feel pain in the lower left abdomen, possibly with fever and changes in bowel habits. For mild cases in otherwise healthy people, the American Gastroenterological Association notes that antibiotics can be used selectively rather than routinely, meaning some episodes resolve with rest and dietary changes alone.
Complicated diverticulitis is a different situation. When a pouch perforates more significantly, it can form a walled-off pocket of infection (abscess) near the colon. If the infection isn’t contained, it can spread into the pelvic or abdominal cavity. In the most serious cases, stool leaks freely into the abdomen, causing widespread infection that requires emergency treatment. Doctors classify these stages on a scale from contained inflammation near the colon wall all the way up to generalized infection of the abdominal lining. The progression from one stage to the next isn’t inevitable, but complicated cases need prompt medical attention.
Recurrent episodes can also lead to chronic complications like narrowing of the colon (which makes it harder to pass stool) or the formation of abnormal connections between the colon and nearby organs. These chronic changes sometimes develop gradually after multiple bouts of inflammation, even if each individual episode seemed manageable.