High Quality Content by WIKIPEDIA articles! Barry James Marshall, AC, FRS, FAA (born 30 September 1951) is an Australian physician, Nobel Prize laureate in Physiology or Medicine, and Professor of Clinical Microbiology at the University of Western Australia. Marshall is well-known for proving that the bacterium Helicobacter pylori (H. pylori) is the cause of most peptic ulcers, reversing decades of medical doctrine holding that ulcers were caused by stress, spicy foods, and too much acid.