system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does "rapid rehydration kill"?

I am reading a book about the nuclear bombs thrown on Hiroshima and Nagasaki. During the blast, the heat skyrocketed and this caused survivors to lose most of the water in their bodies. As a result, they felt extremely thirsty yet they would die when they drank. 

Did they die because they drank? The book says "rapid rehydration kills" but I don't understand why this would be so. I would understand better if it was said they could not drink fast enough but then surely they wouldn't even make it to the watering holes/rivers/water tanks? 

So: why would rapid rehydration kill a dehydrated person?

E: For the record, the book I am reading is "Hiroshima Nagasaki" by Paul Ham.
gpt
Alright, think of it this way.

Your cells are little sacs containing a bunch of solutes, like sodium and electrolytes. When the water-solute balance in your body is off (because you've drunk more water than your body can handle), the cells attempt to compensate by absorbing water from outside the cell (osmosis). The cells figure, *"It's OK! We have enough salts and electrolytes that we can take on a little extra water!"* 

...but if the imbalance is great enough, the cells can't absorb enough water to correct it. All that extra water causes the cells to swell. When the brain cells swell this can interfere with their ability to function or cut off blood flow, resulting in death.