Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 .
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 .