A 45-year-old has burning epigastric pain relieved by food; endoscopy shows a duodenal ulcer. H. pylori testing is positive — the bacterium burrows through mucus, urease neutralizes acid locally, chronic gastritis weakens defense, and acid digests the duodenum. Triple therapy (two antibiotics + PPI) eradicates it — curing most ulcers permanently, unlike antacids that only soothe. The ulcer story flipped in the 1980s: from 'stress and spicy food' to a curable infection — Marshall drank the bacteria to prove it (Nobel 2005).