Helicobacter pylori is a Gram-negative ε-proteobacterium that colonizes about 50% of humans. Some pertinent characteristics are that it can survive the acid of the stomach, produces urease to neutralize it and is motile due to apical flagella. Not surprisingly given its wide distribution, it has long colonized mankind and its genome encodes many features that allows this. Consequently, it frequently has a persistent lifelong association with humans and, differently from most pathogens that are transmitted horizontally, it is preferentially transmitted vertically, often from mother to child. A variety of genes and polymorphisms, both in H pylori and in humans, mediate the complex host-bacterium relationship, and can also determine if and what pathologies will be triggered by the species. H. pylori is naturally transformable, very recombinogenic and has a high mutation rate. Microbiota studies of the stomach have shown it to be an important species with a potentially regulatory role for the gastric microbial community. Likewise, epidemiological work has suggested that, while it clearly increases the risk of peptic ulcers and gastric cancer in some populations, it is also associated with lower risk of esophageal cancer and several other important pathologies. More recently, antibacterial resistant strains have been isolated, posing a problem for public health officials who called for its eradication. Hence, study of H. pylori and how it interacts with us can help revealing mutualistic or pathogenic interactions and the immune response in the digestive niche.