Why do people suffer from altitude sickness after reaching the high altitude regions?

Why do people suffer from altitude sickness after reaching the high altitude regions? How does their body acclimatise after a couple of days?

People, .visiting high altitude regions experience altitude sickness. This is because the body does not get enough oxygen. The person shows symptoms of nausea, fatigue and heart palpitations. After couple of days, the body responds gradually by physiological adaptation. The body compensates low 02 availability by
(i) increasing RBCs production.
(ii) decreasing binding capacity of haemoglobin.
(iii) increased breathing rate.