Helicobacter pylori is a bacterium. It is slightly curved in the form of a helix so it was named Helicobacter. Some of its cousins are longer and you can see the helix better. Click here to see a relative H. pylori looks like this The bacteria are the long squiggly ones, not the little roundish dots.
Most people catch it before they are five or six years old.
The problem is that the human stomach and its immune system can t get rid of it by itself. As a result, helicobacter can colonize the human stomach for decades if not for life. About the only place it has ever been found is the stomach of humans and some monkeys. It has been with humans for a very long time, and has been found in mummies in the Andes for instance. A whole set of cousin helicobacters colonize the stomachs of other mammals.