During the 1940s and 1950s, a series of outbreaks of diarrhea in hospital newborn nurseries were reported in which the etiological agent appeared to be Escherichia coli identified by serotype. These strains became known as enteropathogenic E. coli (EPEC). Although it is generally recognized that these strains are responsible for diarrhea among children under 2 years of age, only now are we beginning to understand how they produce disease.