An analysis carried out in 1994 by the WHO International Agency for Research on Cancer (IARC) resulted in Helicobacter pylori being designated as a Group 1 carcinogen and thus clearly having an association with the development of gastric cancer. In the case of H. pylori, the evaluation was made solely on the basis of epidemiological results. In Japan, in 1993, only 235,000 of the 60 million people with H. pylori had gastric cancer. This represents only 0.4% of the infected population. Each individual reacts in a unique way to H. pylori infection in terms of the inflammatory response. The probability of developing cancer will be determined by environmental factors such as diet, duration of or age at acquisition of H. pylori infection, the virulence of H. pylori strains, and host factors including genetic make-up.