Sometimes visitors who normally live at lower altitudes experience altitude sickness or acute mountain sickness (AMS). The symptoms of high altitude sickness sometime resemble the flu or a hangover and often manifest themselves six to ten hours after ascent and generally subside in one to two days.
The higher you climb above sea level, the less oxygen there is in the air. This can cause a problem with some folks whose bodies aren’t used to working on so little oxygen. If you stay at a high altitude for a week or more, your body gets used to the lower levels and functions properly.