The enigma of Helicobacter pylori infection and gastric cancer.

Although H. pylori has been recognized as a class I carcinogen, incongruence between infection prevalence and cancer incidence has been reported. Holcombe called attention to the high prevalence of infection in the face of low cancer rates, which he called "The African Enigma". Similar observations have now been made in other geographic areas. Gastric carcinoma should be considered an infectious disease, for which the classical epidemiologic model of causality applies. The model proposes that tissue injury inflicted by the infectious agent is modulated by its interactions with host and environmental factors. Although approximately half of the world's population is infected, only a small proportion of people develop cancer. The African enigma is a striking example of the major contrasts in cancer risk among populations with similarly high prevalence of infection. The mechanisms involved in reducing the risk of cancer in infected individuals are explored in this article, which may lead to the design of effective prevention strategies.