Helicobacter pylori is one of humanity's oldest and closest companions, and yet it took scientists more than a century to recognize it. As early as 1875, German anatomists found spiral bacteria colonizing the mucus layer of the human stomach, but because the organisms could not be grown in a pure culture, the results were ignored and then forgotten. It was not until 1982 that Australian doctors Barry J. Marshall and J. Robin Warren isolated the bacteria, allowing investigations of H. pylori's role in the stomach to begin in earnest. Over the next decade researchers discovered that people carrying the organisms had an increased risk of developing peptic ulcers--breaks in the lining of the stomach or duodenum--and that H. pylori could also trigger the onset of the most common form of stomach cancer [see "The Bacteria behind Ulcers," by Martin J. Blaser; Scientific American, February 1996].
Just as scientists were learning the importance of H. pylori, however, they discovered that the bacteria are losing their foothold in the human digestive tract. Whereas nearly all adults in the developing world still carry the organism, its prevalence is much lower in developed countries such as the U.S. Epidemiologists believe that H. pylori has been disappearing from developed nations for the past 100 years thanks to improved hygiene, which blocks the transmission of the bacteria, and to the widespread use of antibiotics. As H. pylori has retreated, the rates of peptic ulcers and stomach cancer have dropped. But at the same time, diseases of the esophagus--including acid reflux disease and a particularly deadly type of esophageal cancer--have increased dramatically, and a wide body of evidence indicates that the rise of these illnesses is also related to the disappearance of H. pylori.