On Water Intoxication

It's summertime and we all love to train outdoors, we ofter sweat a lot and we know the risks of dehydration. We all know that drinking a lot can prevent it.
Dehydration has an opposite physiological status, called overhydration or water intoxication. Basically, it happens when we drink a lot of water in a little time. I won't go in medical details, but it depends on a disequilibrium between the loss of sodium that happens when sweating, and the low sodium intake of drinking water to replace the fluids. It may become a severe status that, if not treated, can lead to coma and death.
This page of Wikipedia explains very well the process of water intoxication. Looking at the reported cases, death by water intoxication happens usually in infants, in people making contests on who drinks more water, or while on drugs (especially extasy). There are cases in marathon and long distance runners that drink too much during the performance. The initial symptoms are very similar to those of dehydratation, so runners tend to drink even more.
To prevent the excessive loss of sodium, it's important to calculate the right salt quantity used in food, or to prefer a salty drink to fresh water.