In the versatile single species of Escherichia coli, the diarrheagenic group displays a remarkable array of virulence traits. These comprise microbial attachment, production of secretory endotoxins or cell-destroying cytotoxins, direct epithelial cell invasion, and localized effacement of the epithelium. The knowledge of how enteric E. coli induce disease has become increasingly important in the world, because of new pathogen emergence, increasing threats of drug resistance, and growing awareness of their importance in malnutrition and diarrhea. Numerous research programs have demonstrated various mechanisms of pathogenesis. We point out how some pathogens are able to develop intercourse with their host through subversion of its cytoskeleton and signaling processes without toxin secretion or heavy invasiveness. In that domain, the cellular biology of infected cells owes fundamental data to the electron microscopic approach. Combined with advances in microbiology and molecular biology, this approach may provide answers to many unanswered questions.