Altitude Sickness is the condition that some people experience after ascending rapidly to heights of more than about 6,500 feet (2,000 m).
Others do not suffer until they reach an altitude of more than 10,000 feet (3,000m).
The condition occurs because air at high altitudes contains less oxygen than at lower altitudes. Individual susceptibility varies greatly; it is most severe in the elderly and those with heart or respiratory problems.

Altitude sickness symptoms:

Altitude sickness symptoms include severe headache, shortness of breath, rapid heartbeat, dizziness, and nausea with diarrhea. Because the reduced supply of oxygen reaching the brain, the person experiences mental confusion and suffers from poor coordination and insomnia.
No person with heart or lung disease should consider trips to high altitudes without first consulting a physician.

Altitude sickness remedies:

In time the body gradually adjusts to the lower oxygen concentration. Physical exertion should be avoided. For severe symptoms, breathing a mixture of pure oxygen and air offers relief.