Helicobacter pylori may be the most successful pathogen in human history. While not as deadly as the bacteria that cause tuberculosis, cholera, and the plague, it infects more people than all the others combined. H. pylori, which migrated out of Africa along with our ancestors, has been intertwined with our species for at least two hundred thousand years. Although the bacterium occupies half the stomachs on earth, its role in our lives was never clear. Then, in 1982, to the astonishment of the medical world, two scientists, Barry Marshall and J. Robin Warren, discovered that H. pylori is the principal cause of gastritis and peptic ulcers; it has since been associated with an increased risk of stomach cancer as well.