Untreated diarrhea will always cause dehydration to a certain degree. Sometimes, it’s just a mild problem that can be resolved with a bit more water intake. In some cases, it requires the intake of Oral Rehydration Solutions, and in particularly severe cases, hospital treatments are necessary. Children are particularly vulnerable to dehydration, and the younger the child, the greater the risk of severe dehydration. Dry mouth, thirst, headache and decreased urine output are the signs of dehydration.