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.
Marshall was born in Kalgoorlie, Western Australia and lived in Kalgoorlie and Carnarvon until moving to Perth at the age of eight. His father held various jobs, and his mother was a nurse. He is the eldest of four siblings. He attended Newman College for his secondary education and the University of Western Australia School of Medicine, where he received a Bachelor of Medicine, Bachelor of Surgery (MBBS) in 1974. He married his wife Adrienne in 1972 and has four children.
In 1979, Marshall was appointed Registrar in Medicine at the Royal Perth Hospital. He met Dr. Robin Warren, a pathologist interested in gastritis, during internal medicine fellowship training at Royal Perth Hospital in 1981. Together, they both studied the presence of spiral bacteria in association with gastritis. In 1982, they performed the initial culture of H. pylori and developed their hypothesis on the bacterial cause of peptic ulcers and gastric cancer. It has been claimed that the H. pylori theory was ridiculed by established scientists and doctors, who did not believe that any bacteria could live in the acidic environment of the stomach. Marshall was quoted as saying in 1998 that "everyone was against me, but I knew I was right." On the other hand, it has also been argued that medical researchers showed a proper degree of scientific scepticism until the H. pylori hypothesis could be supported by evidence.