Helicobacter pylori are gram negative bacteria that live in the human stomach. They may be considered" slow" bacterial pathogens because of their ability to persist in this seemingly hostile environment for decades if not for life, because they induce an inflammatory response, and because this interaction may lead to a variety of clinical consequences, among them peptic ulceration and gastric neoplasia. It is important to understand the biology of H. pylori infections, not only because it is involved in the pathogenesis of two of the most important diseases of the upper gastrointestinal tract, peptic ulcer disease and adenocarcinoma of the stomach, but because it also serves as a model of the effects of chronic mucosal inflammation on endocrine homeostasis and on oncogenesis. However, H. pylori infection also allows us to view the consequences of persistence from the context of the successful parasite as well. We know that, if the range of a microbe is limited to a single species or a closely related group of species, there is a strong negative selective pressure against those that are overwhelmingly virulent; the central concern of the single reservoir microbe is the ability to be transmitted from host to host (1). The organism must strike a balance with regard to its virulence, such that a microbial density is produced that maximizes transmission in the normal host. All such balances involve costs (to the microbe); one cost is that, when a host with suboptimal defenses is encountered, disease may ensue that limits the ability of the microbe to find a new host. H. pylori appears to havesuccessfully met thesechallenges. At least a third of the world's human population is infected (2), usually beginning early in life, and the major disease conse-quences appear to be expressed years or decades after infection is acquired (3), diminishing negative selection against infection. This success probably was not won overnight, but rather reflects a long evolutionary history of coexistence of humans and their gastric bacterial parasites. Persistent infections, espe-cially those in which an immune response indicates that the host is aware of the microbe's presence, must have a complex