Dehydration occurs when your body loses too much fluid. When you stop drinking liquids or lose large amounts of fluids because of diarrhea, vomiting, or sweating, the body reabsorbs fluid from the blood and other body tissues. Look for signs of dehydration:
Dehydration is very dangerous for infants, small children, and older adults. Watch closely for its early signs when they have been out in the sun. It is most dangerous for newborns, so it is important to keep babies less than 6 months old out of the sun.
By the time you become severely dehydrated, there is no longer enough fluid in the body to get blood to the vital organs and you may go into shock, which is life-threatening and requires emergency medical treatment. Look for signs of shock:
WebMD Medical Reference from Healthwise