What will happen if diverticulitis goes untreated?

Treat diverticulitis: Technically diverticulitis is a microperforation of the colon at the site of the diverticulum. However if left untreated, it will likely progress to a frank perforation of the sigmoid colon potentially leading to abdominal sepsis and death. In short, if uncomplicated, most cases can be treated with antibiotics and nothing by mouth for a few days.

Quite variable: If it's a mild case, possibly nothing and your body may health without any intervention. But the opposite side of the spectrum is that you can develop life-threatening complications include perforation of your colon and complicating sepsis. In those actively being treated and requiring surgery, some studies show up to a 5% chance of death!