As we climb to higher altitudes, much changes in the environment. Apart from the fact that the air is less polluted and the trees make way for rocks and snow, the composition of the air also changes. One of these changes is that the oxygen partial pressure is getting lower. So there are fewer oxygen molecules in the same volume of air than at a lower altitude. The annoying thing is that the climber needs a lot of this to get up the mountain. However, the body can cleverly deal with the reduced oxygen tension by adapting in various ways to still be able to take in enough of this gas. However, these adaptations are not always sufficient or efficient enough, which can cause altitude sickness. The main cause of altitude sickness is therefore the reduced partial pressure of oxygen in the air.
There is nothing you can do about the changing environment. However, as mentioned earlier, the body can adapt to it in miraculous ways. The only crucial condition is that it needs time to do so. This is the main culprit for contracting altitude sickness: not giving the body the time to get used to it, or in other words, not acclimatising adequately.
Between 1500 m and 5500 m, a person is able to fully acclimatise. From 5500 m the adaptive capacity is insufficient: the body is no longer able to fully adapt to the reduced partial oxygen tension. From that altitude, an individual will therefore also be much more susceptible to altitude sickness. A climber can expect to start showing symptoms from very high altitudes.
One of the tricky aspects of getting altitude sickness is the physiology of the individual. The tricky part is that this works differently from person to person, which means that the acclimatisation process will also be different. This makes it difficult to say at what point and to what extent a climber will become altitude sick. A factor in the occurrence of altitude sickness is the functioning, the adaptability to altitude, of one's own body.