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Barry Marshall was awarded the prestigious Nobel Prize in Physiology or Medicine in 2005 for his groundbreaking work on the bacterium Helicobacter pylori. His research revolutionized the understanding of stomach ulcers, which were previously thought to be caused by stress and spicy foods. Marshall's discovery that H. pylori was the primary culprit led to the development of effective treatments for ulcers, significantly improving the lives of millions worldwide.