barry james marshall, ac, frs, faa (born 30 september 1951) is an australian doctor and winner of the 2005 nobel prize in physiology or medicine. he is professor of clinical microbiology at the university of western australia. marshall is well known for proving that bacteria "helicobacter pylori" are the cause of most stomach ulcers. this changed years of medical belief which said that ulcers were caused by stress, spicy foods, and too much acid. he has recently taken a part-time position at the pennsylvania state university.