The most noteworthy recent developments regarding infections of the intestinal tract have been the emergence of old bacterial foes in new guises. These developments include the identification of a new serotype of toxigenicVibrio choleraewith the potential to cause epidemics, the recognition of novel varieties of diarrheagenic strains ofEscherichia coli, and the emergence ofSalmonellaandShigellaspecies resistant to most clinically useful antibiotics. On the other hand, we are witnessing the onset of a new age of immunization based on genetically engineered vaccines which after a single oral dose will afford long-lived mucosal immunity to infection. These exciting discoveries have been accompanied by an improved understanding of the pathogenesis of bacterial diarrhea and its systemic sequelae. The aim of this review is to highlight some of these developments.