Helicobacter pylori colonizes the human stomach, especially during childhood. However, a variety of H. pylori strains exists, with major differences in virulence characteristics which probably account for different clinical symptoms, and the majority of infected subjects remains asymptomatic. Helicobacter pylori infection is correlated with socioeconomic conditions and hygienic circumstances, resulting in an extremely high prevalence in children in developing countries. Commercial screening tests are not capable of separating the more virulent strains (type I with vacuolating toxin VacA and CagA protein) from the less virulent strains (type II, VacA and CagA negative). Type I strains, but not type II, are associated with an increased risk for duodenal ulcer and gastric cancer. Therefore, future screening tests and vaccinations should focus on the type I strains.