Helicobacter pylori infection is the cause of chronic gastritis that progresses to atrophic gastritis over years and decades in more than half of affected individuals. H. pylori gastritis and, particularly, subsequent atrophic gastritis increase the risk for gastric cancer on multifactorial basis. Largely unknown cascades of manifold reactions result in gene errors of epithelial cells in gastric and atrophic stomach, which raise the likelihood of gastric neoplasias and cancer among people infected by H. pylori. The prevalences and incidences of gastric cancer and H. pylori are similarly decreased during the past decades in western countries, supporting the view that H. pylori infection is a key event and a trigger of the phenomena that result in cancer in some of the infected subjects.