Helicobacter pylori is considered one of the most common pathogenic infections of mankind. Despite its worldwide distribution, the pathogenesis of H pylori associated gastroduodenal disease remains poorly understood. What is clear is that only a minority of infected individuals develop severe inflammation leading to peptic ulcers or gastric cancer, the more severe manifestations of helicobacter infection. What are the factors which determine whether an infected individual will develop severe disease? It has been suggested that phenotypic or genotypic differences among bacterial isolates may be important in disease.1 There is evidence to suggest that individuals infected with strains of H pylori which express the cytotoxin associated gene product CagA, a marker for the presence of a “pathogenicity island”, are more likely to develop peptic ulcers or gastric cancer.2-5Recent observations suggest that polymorphism ofvacA genotypes may determine whether anH pylori infected individual develops gastritis or an ulcer.6 However, only a small fraction of the estimated 60% of all individuals infected with such strains ofH pylori develop severe gastroduodenal disease (peptic ulcers or gastric cancer). This suggests …