The human stomach is lined with a protective coating of mucous. The acids that digest our food are very strong, and without this protective lining, they would erode the walls of the stomach. Mucous production is dependent on blood flow to the stomach tissue; undue stress sends messages to the brain to reduce blood flow (hence, some people are "white as a sheet" when they're frightened) and therefore stomach secretions are reduced, leaving the stomach open to attacks from its own digestive enzymes.

So went the medical logic for many years, but an Australian doctor named Barry Marshall didn't quite buy this stress theory. After years of studying the stomach biopsies of ulcer patients, Dr. Marshall presented a paper stating that a previously unidentified bacterium, Helicobacter pylori, was the actual cause of ulcers.

His theory was met with skepticism from the medical community, so Dr. Marshall took a bold step — he drank a huge dose of h. pylori and very quickly developed an ulcer. He then cured himself with a regimen of antibiotics. Today, it is universally accepted that ulcers are caused by this bacterium, and antibiotics are the standard treatment. Recently, scientists have also linked h. pylori to stomach cancer and heart disease, and are studying the relationship in hopes of finding new treatments for both conditions.