Diarrhoea causes a person to lose both water and electrolytes. Electrolytes are the salts such as sodium and potassium which are essential to the functioning of every cell in the body. If vomiting accompanies the diarrhoea then these losses are accelerated.

The loss of water and electrolytes leads to dehydration. This happens especially quickly in a child. The child becomes irritable, and its mouth is dry. Its mother may notice that it is passing urine less frequently, and when it cries there may be no tears. As dehydration progresses, a nurse or doctor will notice other signs as well.

The skin turgor is decreased, which means that when a fold of skin is pinched up from the abdomen it stays raised up for a moment, even after it is let go. Also, the eyes sink down into the orbits. The pulse becomes rapid and weak. By this stage, the internal organs are all suffering. They lack both water and electrolytes, and begin to fail.

The child is in shock and falls into a coma, and soon after that the heart stops beating, which is termed cardiac arrest. Death from dehydration usually occurs when 10 to 15% of the total body weight is lost. In severe diarrhoea such as that caused by cholera this may take only a couple of hours.