Diarrheal illness remains one of the leading causes of morbidity and mortality worldwide, despite ongoing progress in our basic understanding of its epidemiology, pathogenesis, and treatment. In developing areas, where access to safe drinking water and sewage disposal are often limited or even absent, infectious diarrhea is a major cause of childhood mortality, with an estimated 1.9 and 5.6 million deaths among children globally per year (ie, 15000 and 15,000 children dying each day) because of diarrhea and malnutrition, respectively [1]. Beyond this, however, are the underrecognized longterm effects of frequent early childhood diarrheal episodes, which include permanent shortfalls in physical and cognitive development, with decrements of up to 8 cm in growth, 10 intelligent quotient points, and 12 months of schooling attributable to early childhood diarrhea and enteric parasitic infection [2–5]. In developed countries, where modern sanitation practices are commonplace, the burden of early childhood diarrhea has been dramatically reduced. Nonetheless, recent large outbreaks of infectious diarrhea due to Cryptosporidium species, enterohemorrhagic Escherichia coli, and

Clostridium difficile underscore the fact that major intestinal infections lurk nearby even in the cleanest environments. Moreover, sporadic diarrheal illness remains important in developed countries, with 1–2 episodes per person per year, second only to the common cold as the most common infectious illness. Two articles presented in this issue of