H. pylori is a super common gut bacteria: More than half of all humans carry it. It's also responsible for 80% to 90% of all stomach cancers, making it the world's second leading cause of cancer mortality. But not everyone that has H. pylori gets stomach cancer - it's rare in Africa, even though lots of people have the microbe. So why does H. pylori cause cancer in some groups but not others? Researchers may finally have an answer.