What is high altitude sickness ? Write its symptoms.

If one had ever been to any high altitude place $(>3,500 \mathrm{~m}$ like Rohtang Pass near Manali and Mansarovar in Tibetan Autonomous Region), the pathological effect caused by acute exposure to low partial pressure of oxygen at high altitude is called altitude sickness. Its symptoms include nausea, fatigue and heart palpitations.