In 1984, Australian physician Barry Marshall swallowed a cloudy broth teeming with live Helicobacter pylori bacteria to prove that stomach ulcers were caused by infection, not stress or spicy food. At the time, the medical establishment insisted the stomach was too acidic to host any microorganism, and treating ulcers as a lifelong condition was a lucrative industry.

Working with pathologist Robin Warren, who first spotted the spiral bacteria in stomach biopsies, Marshall used self-experimentation to fulfill Koch's postulates. Within days he developed gastritis, then cured himself with antibiotics. Their work redefined ulcers as an infectious disease and earned the 2005 Nobel Prize in Physiology or Medicine.

• The reigning belief held the stomach was sterile, so bacterial theories of ulcers were dismissed as impossible.
• The ...