There are six and a half billion human stomachs on this little planet of ours, and over half of them are home to a microbe called Helicobacter pylori. Scientists have known about the bacteria since the late 1800s, but it wasn’t until the 1980s that Australian doctors noticed that H. pylori was in the stomachs of just about everyone with an ulcer. A swig of antibiotics turned out to be a great way to make ulcers disappear. Scientists have since demonstrated that H. pylori strikes up an uneasy truce with its human hosts. In most cases, H. pylori lives amicably in our stomachs. When the truce is broken, the microbe triggers a cascade of reactions that leave a stomach cratered.