Much of the developing world's population lives in substandard housing, under crowded conditions, without piped water or sanitation. Under these conditions of pervasive fecal contamination, the various bacterial, protozoal, and viral agents that cause diarrheal illness are readily transmitted (1–6). In low-socioeconomic-level populations in the developing world, infants and toddlers experience from 4–10 episodes per child per yr during the first 2 yr of life (1–3). Up to 20% of life experience of infants may be spent suffering from diarrheal illness (1–3) and up to one-third of deaths among children <2 yr of age are due to diarrheal dehydration, persistent diarrhea, and other complications of diarrheal illness (7).

A striking feature of pediatric diarrheal illness in developing countries is the large proportion associated with bacterial enteropathogens. Among the most important agents are enterotoxigenic Escherichia coli , enteropathogenic E. coli, and Shigella (1–6, 8–11). Enteroaggregative E. coli, a recently described category of diarrheagenic E. coli has been incriminated as an important cause of persistent diarrhea in developing countries (12–15). Consequent to clinical infections caused by the various agents and their antigenic varieties, infection-derived immunity is acquired (4, 16–18); this is reflected in markedly lower incidence rates in older children and adults and in increased prevalence of antibody with age (18).

Quite distinct patterns of diarrheal disease are encountered in industrialized regions of the world, where there is access to microbiologically monitored drinking water, flush toilets, wastewater treatment, and adequate housing with little or no crowding. In these relatively affluent settings, viral agents of diarrhea predominate in pediatric populations (19). Nevertheless, some notable exceptions occur in specific settings in which hygienic conditions are less adequate (e.g., pediatric day care, custodial institutions for the mentally retarded), with the net effect of creating environments similar to those observed in less developed countries (20, 21).

1. Urbanization. At one extreme, we may consider the diarrheal disease problem among the most isolated and primitive populations