There is less oxygen at altitude; it should be no surprise that it causes people to feel unwell. How this shortage of oxygen leads to altitude sickness is still not fully understood. Many scientists believe it is due to swelling of the brain caused by increased blood flow.
Two things are certain to make altitude sickness worse. 1) Ascending to altitude quickly – such as in a flight from sea level to Aspen or Vail. 2) Exercising before full acclimatization – exactly what most do on ski vacations. Physically fit individuals can get altitude sickness as well as the unfit.