As part of this study, the investigators will derive a new clinical prediction rule for severe dehydration in children with diarrhea in the developing world. In addition, the investigators will validate and assess the reliability of ultrasound of the inferior vena cava (IVC) for predicting severe dehydration in children with diarrhea. Finally, the investigators will compare the accuracy of each of these new diagnostic tools to that of the WHO scale, the current standard for assessing dehydration in most low-income countries.