Altitude sickness can occur when the body can't get enough oxygen from the air at high altitudes. This may occur from ascending too fast or exerting yourself too much during a high altitude climb. Symptoms include headache, lightheadedness, loss of appetite, breathlessness while resting, fatigue and sleep interruptions. Severe forms of this illness can be deadly and must be treated immediately. To treat altitude sickness, simply move to a lower altitude--though with milder cases you may rest at the same altitude while drinking plenty of water. To avoid altitude sickness, ascend the mountain slowly and limit physical activity in the beginning of the climb.