In 1983, a pair of Australian microbiologists suggested that gastritis and peptic ulcers were infectious diseases, contradicting long-held beliefs concerning their epidemiology, pathogenesis, and treatment. In the same year, the 10th edition of Harrison’s Principlesof Internal Medicine described peptic ulcers as due to an unfavorable balance betweengastric acid – pepsin secretion and gastric or duodenal mucosal resistance. Underlying causes cited included genetic and lifestyle (smoking) as well as psychological factors (anxiety, stress). Treatment with bismuth salts, antacids, and inhibitors of acid secretion gave relief but not cure. Relapsing patients (50 to 80%) were subjected to surgical treat-ments (vagotomy, partial gastrectomy), which had their own set of complications (reflux, afferent loop syndrome, dumping syndrome). All of this was logical and supported by clinical observations and research studies. It was simply incorrect. The bacteria now called Helicobacter had been observed but dismissed because they were so common and its urease was once considered a secretory product of the stomach itself. The paper by Warren and Marshall (see Additional Reading) stimulated the reversal, which has led to cures using antimicrobics and new ideas linking Helicobacter infection to cancer. This experience has also left us with a sense that we can never be smug about what we “know” in medicine.