to lose water from the tissues of the body. When babies have diarrhea or are vomiting, they can lose large amounts of water and salts. If these fluids are not replaced, a child becomes dehydrated. Signs of dehydration in a baby include a dry mouth and sticky saliva, dark yellow urine, less urine, a fast heartbeat and low energy. Severe dehydration can be dangerous, in some cases, even fatal.