Although certain factors appear to predispose the host to infection by Helicobacter pylori, clearly the bacterium possesses a well-defined battery of virulence factors that allow the organism to: (1) colonize the gastric mucosa (urease, flagella, adhesins, acid-inhibitory protein, iron acquisition proteins, and heat shock proteins); (2) evade host defense (shedding of surface proteins, catalase, superoxide dismutase, and poorly reactive lipopolysaccharide); and (3) damage host tissue (vacuolating cytotoxin, protease, CagA-related factors, inducers of cytokines, and chemotaxins). Together these factors allow H. pylori to persist in the host, establishing a chronic infection. Although many of these virulence factors are produced by all strains of H. pylori, there are also well-defined pathogenicity islands (contiguous stretches of chromosomal DNA) present in some strains that encode additional proteins including CagA that potentiate virulence. Strains possessing these "virulence cassettes" are isolated more frequently from patients with the more serious clinical manifestations associated with duodenal ulcer than from patients with gastritis alone or nonulcer dyspepsia.