For a long time, peptic ulcers (ulcers in the lining of the stomach or duodenum) were primarily attributed to lifestyle factors like stress and diet leading to excessive acid production. However, research by Barry Marshall and Robin Warren in the 1980s revolutionized the understanding of peptic ulcers by demonstrating that the bacterium Helicobacter pylori is a major cause. This bacterium can survive in the acidic environment of the stomach and colonize the gastric lining, causing inflammation and making the tissue vulnerable to acid damage, leading to ulcer formation.