After Dr. Marshall and Dr. Warren discovered the role of the spiral-shaped H. pylori bacterium, they and others conducted trials showing that antibiotics and drugs inhibiting the production of stomach acid could cure gastritis and most stomach and duodenal ulcers.

The inflammation produced by H. pylori can also lead to certain stomach cancers that seem to be prevented by antibiotic treatment of the bacterium. In the early 1900's, stomach cancer was a leading cause of cancer deaths in the United States. But its incidence fell significantly, for unknown reasons, before the discovery of H. pylori's role. Stomach cancer remains the second leading cause of cancer deaths worldwide.

H. pylori is also an important player in a type of lymphoma cancer of the stomach known as MALT, for mucosa-associated lymphoid tissue. Such lymphomas usually regress when antibiotics rid the stomach of H. pylori. Since the late 1800's, many doctors had noted the bacterium's presence in the stomachs of patients with ulcers and gastritis, but they ignored the connection.

In the early 1980's, Dr. Warren noted the bacterium in the lower part of the stomach in about half of the patients who had biopsies. He made a crucial observation that signs of inflammation were always present in the surface lining of the stomach near where he observed the bacterium.

Dr. Marshall joined Dr. Warren in studying biopsies from a series of patients. After several attempts, Dr. Marshall succeeded in growing a bacterium that was unknown then; he named it Campylobacter pyloridis, believing that it was a member of the Campylobacter family. (It was later found to be a member of the Helicobacter family and renamed H. pylori.)

Still, many doctors were unconvinced by the findings, a point recognized by the Nobel committee, which said the award went to Dr. Marshall and Dr. Warren "who with tenacity and a prepared mind challenged prevailing dogmas."

Dr. Marshall carried on a medical tradition in experimenting on himself to test his and Dr. Warren's theory and to show that Helicobacter was the primary cause of gastritis, not a secondary invader.