The acquisition of Helicobacter pylori is the main cause of chronic gastritis in humans. In Europe, a small proportion (less than 1%) of gastritis cases are caused by H. Heilmannii, and somewhat more (5%) are autoimmune in origin, in which condition H. pylori may not probably play a role. Recent findings on chronic gastritis and H. pylori acquisition in developed countries can be summarized as: (1) H. pylori gastritis is acquired in childhood and adolescence (age less than 20) in more than 50% of cases; (2) the risk and rate of acquisition is highest in early childhood, after which the rate exponentially declines; (3) new infections occur in adulthood but are quite rare (annual incidence 0.4%, on average, in Finland); (4) H. pylori gastritis is a birth cohort-related phenomenon; i.e., different cohorts show a rate and prevalence of H. pylori gastritis that varies between cohorts; (5) the rate and risk of H. pylori infection is high in cohorts born in the beginning of the century, but is much lower in those born later; (6) this decline is due to a decrease in the rate and risk of H. pylori acquisition in childhood in particular. H. pylori gastritis-related complications, such as peptic ulcer diseases and gastric cancer, show epidemiological features similar to H. pylori gastritis. Both peptic ulcer and gastric cancer have declined in incidence over time. Gastric cancer is a birth-cohort phenomenon in the same way as is H. pylori gastritis, and the incidence of gastric cancer shows a positive but exponential relationship with the "birth-cohort-specific" prevalence of gastritis in the general population.