I read "Into Thin Air" and figured out mountain climbing wasn't for me. If the cold doesn't kill you then you can die from either lack of oxygen or even just the high altitude.
I know two people that died and another who developed cerebral oedema over the course of two hours at much lesser altitudes in the Everest region.
Of the two deaths, one was a 22 year old kid, the other a 54 year old Austrian guy with years of mountaineering experience. Both died in the same week on the 18,500 foot Cho La Pass that connects Gokyo Valley to the Khumbu Valley. The older guy had an apparent heart attack, the younger guy I don't know for sure but everyone said he was going up way too fast without proper acclimitization.
As an aside, they had to pay a Sherpa some obscene amount of money to bring a yak up to the Pass and recover the body (Sherpas don't like hauling dead bodies around).
The third was a woman in our little trekking group who went from a minor to a splitting headache, and then vomiting, in a couple hours. It was pretty obvious what was happening to her, so her boyfriend took her down from Lobuche to the lower-altitude next town where she recovered almost instantly.
Going down several hundred meters is usually (not always) enough to fix the problem, if you catch it early enough.