About a decade ago, Michael Valenti of Woodstock, N.Y., suddenly developed severe pain in the lower right side of his abdomen. He had a slight fever and thought he had appendicitis. So did his doctor, who sent him to the emergency room, where an M.R.I. revealed not appendicitis but diverticulitis, an infection in an outpouching of his colon he never knew he had.

The attack came “out of the clear blue,” Mr. Valenti told me, and was especially confusing because the vast majority of diverticular pouches and resulting infections develop in the sigmoid colon located in the left lower abdomen.

Diverticulosis, as the diverticular pouches are called, is very common in Western countries, where the typical overly refined diet is low in fiber, and the risk of developing it increases with age, reaching 60 percent by age 70. Unless the pouches are discovered in the course of an unrelated exam, like a colonoscopy, most people with diverticulosis don’t know they have it.

However, the pouches can trap food residue as it passes through the colon and become perfect breeding grounds for bacteria. As a result, about 20 percent of people with diverticulosis will develop diverticulitis, an inflammation and infection that if not adequately treated can result in dangerous abscesses, perforation of the colon and peritonitis.