For most of the 20th century, medical dogma said that peptic ulcers--painful erosions in the stomach--were caused by stress and spicy foods. That all changed in 1984, when Austrian researchers Barry Marshall and J. Robin Warren discovered spiral-shaped bacteria in gut biopsies of most people with ulcers. (Nobody believed them at first, so Marshall convinced a colleague to give him a biopsy before and after swallowing a culture dish of bugs harvested from one of his patients. Within five days, he was vomiting and had halitosis, and the biopsy confirmed an ulcer.) The duo won the Nobel prize for their work in 2005.

The fingered bacteria was Helicobacter pylori, and since that discovery, many researchers have hoped to eradicate the species with drugs or vaccines. But that's probably not a good idea. About half the world's population carry H. pylori, most without any gastric symptoms. In 2008, researchers from the National Cancer Institute suggested that these bacteria may help fend off throat cancer. In a review of 19 studies, the scientists found that people who carry a certain strain of H. pylori are nearly half as likely to get adenocarcinoma of the esophagus than are those who don't carry it.

The scientists aren't sure how the protective effect might work. It could be that the bacteria reduce the amount of acid in the stomach, which curbs acid reflux, a known risk factor of throat cancer. Or it could work by preventing another cancer risk factor, obesity: H. pylori damps down ghrelin, a hormone that makes us hungry.