What causes babies to be dehydrated?

Vomiting, Diarrhea. Although there are other causes of dehydration, such as poor eating or overheating, most cases in the USA result from vomiting and/or diarrhea. Dehydration results when the baby is losing more fluid than she is taking in. Treatment consists of oral rehydration solutions (i.e. Pedialyte) and sometimes IV fluids. If you feel your baby may be dehydrated, call your pediatrician.
Lack/loss of fluid. Dehydration applies when fluids are not replased when lost in normal body activities like sweating, urinating or lost to breating (think fog breath in cold weather). Additional losses occur with vomiting or diarrhea. Babies often refuse to drink if they think they will vomit again or eat if it causes cramps & diarrhea. Weather a virus or other issue, babies may refuse fluids and dehydrate.