Original: Marshall is well-known for proving that bacteria Helicobacter pylori are the cause of most stomach ulcers, reversing decades of medical doctrine which held that ulcers were caused by stress, spicy foods, and too much acid.
Simplified: 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.