Real-time microscope footage of a Helicobacter pylori bacterium. H. pylori is a motile bacterium that moves by beating its flagella. It is a leading cause of gastritis and stomach ulcers, as it can colonise the stomach lining, leading to its weakening and eventually ulceration. Infection with H. pylori is also a risk factor for gastric carcinoma. H. pylori is a common member of the gastrointestinal flora, and it is present in more than half the world's population, with the majority never developing complications. The bacterium is around 3 micrometres in length.