the bends.

"The symptoms of altitude sickness," wrote my worried father, "are brought on entirely because of the elevation and thin oxygen content in the air. As a result, the brain tells the body to hyperventilate (unconsciously) to bring in more oxygen. Don't push yourself. If you don't feel well, find the quickest way to descend."
Tibet lies somewhere in the ballpark of 4,000m above sea level. I had virtually no context for regarding altitude prior to this trip, only a mounting sense of paranoia towards a condition I assumed (like just about everything else) could be shrugged off with a hot shower and a long nap. The fear was chiefly brought on by the assiduous preparations being taken by a couple members of my young, fit, globe-trotting group. They ate, beginning three days prior to arrival, exclusively fruit and bread. They took their daily Diamox; they drank water perfunctorily and obsessively. And, perhaps as a result of too much preemptive mental stress, which can increase the heart rate, which at high altitudes can induce additional oxygen depletion, they got sick.
The rest of us escaped with mild headaches, which evaporated after the third day.