Infection-associated cancers account for a large proportion of human cancers, and gastric cancer, the vast majority of which is associated with Helicobacter pylori infection, is a typical example of such cancers. Epigenetic alterations are known to occur frequently in gastric cancers, and H. pylori infection has now been shown to induce aberrant DNA methylation in gastric mucosae. Accumulation of aberrant methylation in gastric mucosae produces a field for cancerization, and methylation levels correlate with gastric cancer risk. H. pylori infection induces methylation of specific genes, and such specificity is determined by the epigenetic status in normal cells, including the presence of H3K27me3 and RNA polymerase II (active or stalled). Specific types of inflammation, such as that induced by H. pylori infection, are important for methylation induction, and infiltration of monocytes appears to be involved. The presence of an epigenetic field defect is not limited to gastric cancers and is observed in various types of cancers. It provides translational opportunities for cancer risk diagnosis incorporating life history, assessment of past exposure to carcinogenic factors, and cancer prevention.