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. Until that discovery, for which the men shared a Nobel Prize, in 2005, stress, not an infection, was assumed to be the major cause of peptic ulcers.
H. pylori is shaped like a corkscrew and is three microns long. (A grain of sand is about three hundred microns.) It is also one of the rare microbes that live comfortably in the brutally acidic surroundings of the stomach. Doctors realized that antibiotics could rid the body of the bacterium and cure the disease; treating ulcers this way has been so successful that there have been periodic discussions of trying to eradicate H. pylori altogether. The consensus was clear; as one prominent gastroenterologist wrote in 1997, “The only good Helicobacter pylori is a dead Helicobacter pylori.” Eradication proved complicated and expensive, however, and the effort never gained momentum. Yet few scientists questioned the goal. “Helicobacter was a cause of cancer and of ulcers,’’ Martin J. Blaser, the chairman of the Department of Medicine and a professor of microbiology at the New York University School of Medicine, told me recently. “It was bad for us. So the idea was to get it out of our bodies, as fast as we can. I don’t know of anyone who said, Gee, we better think about the consequences.”