Dehydration is the loss of water and salts essential for normal body function. It occurs when the body loses more fluid than it takes in.

Young and middle-aged adults who drink when thirsty generally maintain fluid balance. Children need more water because they expend more energy, but most children who drink when thirsty get as much water as they need. Adults over the age of 60 who drink only when thirsty probably get only about 90% of the fluid they need.

Dehydration is a major cause of infant illness and death. Dehydration can occur in children who have stomach flu characterized by vomiting and diarrhea, or who cannot or will not take enough fluids to compensate for excessive losses associated with fever and sweating of acute illness. An infant can become dehydrated only hours after becoming ill.

When the body's fluid supply is severely depleted, hypovolemic shock is likely to...