[rgmerk]

Both of the above, but also your body's ability to adapt to high altitude, which isn't just a question of general cardiovascular fitness.

If you build up to climbing Everest more gradually you find this out in places and at altitudes where you can be saved.

That said, Everest is an extremely dangerous place even for experienced mountaineers, though it's not the most dangerous . The second highest peak, K2, is so dangerous that roughly one person dies for every eight or so who reach the summit.