Barry James Marshall AC FRACP FRS FAA (born 30 September 1951) is an Australian physician, Nobel Prize Laureate in Physiology or Medicine, Professor of Clinical Microbiology and Co-Director of the Marshall Centre at the University of Western Australia. Marshall and Robin Warren showed that the bacterium Helicobacter pylori (H. pylori) plays a major role in causing many peptic ulcers, challenging decades of medical doctrine holding that ulcers were caused primarily by stress, spicy foods, and too much acid. This discovery has allowed for a breakthrough in understanding a causative link between Helicobacter pylori infection and stomach cancer.