I felt an uncomfortable fullness after the evening meal," recalls Dr. Barry Marshall, "and then woke up at 5 a.m., vomiting." During that week in 1984, the Australian physician, then a medical fellow at a hospital in Perth, was suffering for the first time from gastritis, an inflammation of the stomach often associated with peptic ulcers.

But Marshall's discomfort was mixed with elation because he knew the source of his illness: a mixture of bacteria he had taken from the stomach tissue of a patient with a suspected ulcer, carefully cultured and then voluntarily swallowed himself. By acting as his own guinea pig, he had proved to his satisfaction that these bacteria, later called Helicobacter pylori, could bring on ulcers and gastritis.

Others were not convinced. Medical doctrine at the time held that gastritis and ulcers in the stomach or intestines were caused by excess acid, brought on partly by stress or diet. Normal acidic gastric juices were believed to keep the stomach sterile. "No one thought bugs could live there," says Marshall. But he and Robin Warren, the pathologist who discovered the culprit bacterium, had found that the bugs survived by burrowing under the stomach's mucous lining, which shielded them from the acid.

The medical community, skeptical of Marshall's ideas and critical of his unorthodox self-experiment, largely rejected his theory. For a few years, he says, "I was a lone voice." But he began treating ulcer patients with antibiotics and a coating agent and was soon achieving a 75% cure rate. Other doctors then tried variations of Marshall's prescription, got even higher cure rates and helped confirm that, except for cases in which drugs such as aspirin are the culprit, H. pylori is the leading cause of ulcers. That helped Marshall gain worldwide, if belated, recognition.

In 1994 the U.S. National Institutes of Health made that recognition official when it recommended antibiotics for the treatment of ulcer patients who have the bacteria. Marshall has since received several prizes, including the Lasker Award in 1995, and is now a professor at the University of Virginia, where he founded a Helicobacter and intestinal immunology research center. He helped develop tests for H. pylori--which may also be implicated in some stomach cancers--and is now working on formulating simpler tests. Encouraged by the recent deciphering of H. pylori's genome, he is also continuing work on developing a vaccine to combat his favorite bacteria.

--Reported by Sabrina Yohannes/New York