At first, doctors didn’t believe that bacteria could live in the stomach at all. Too acidic, they thought. But in 1984, a young Australian physician named Barry Marshall gulped down an infamous concoction of beef broth laced with Helicobacter pylori bacteria. On day eight, he started vomiting. On day 10, an endoscopy revealed that H. pylori had colonized his stomach, their characteristic spiral shape unmistakeable under the microscope. Left untreated, H. pylori usually establishes infections that persist for an entire lifetime, and they’re common: Half of the world’s population harbors H. pylori inside their stomach, as do more than one in three Americans. In most cases, the microbe settles into an asymptomatic chronic infection, but in some, it becomes far more troublesome. It can, for example, cause enough damage to the stomach lining to create ulcers. Worse still, H. pylori can lead to cancer. This single bacterium is by far the No. 1 risk factor in stomach cancers worldwide. By one estimate, some 70 percent can be attributed to H. pylori. But what still puzzles …