Acute mountain sickness or "AMS" is a serious medical condition arising from too little oxygen / too much carbon dioxide in the body. It is brought on by travelling too quickly to a certain altitude, and/or performing physical exercise at altitude when unacclimatised. To say it is unrelated to fitness is only partially true; tolerance to altitude varies greatly amongst individuals, but a fit person will not strain their cardiovascular system as much as an unfit person for the same amount of exercise, so will therefore be at a lower risk of developing AMS. Even a very fit person however is still very likely to develop AMS if they attempt Mont Blanc without some days spent acclimatising.

It is also a mistake to think than AMS is the result of a gradual and linear worsening of mild altitude symptoms such as breathlessness and headache. AMS is in fact a sudden and dramatic onset of symptoms (particularly nausea/vomitting, ataxia (loss of balance), and a complete loss of energy), and leaves the person hardly able to walk or look after themselves. It develops suddenly in around half an hour (often preceeded by repeated yawning) and in that time will almost completely incapacitate the sufferer (symptoms reverse equally dramatically on rapid descent, e.g. in a helicopter - most people have recovered in the ten minutes it takes to reach the valley!).

A six day program allows most people to acclimatise sufficiently to climb Mont Blanc, but less than that and the chance of developing AMS rises significantly. It is perhaps a general lack of understanding of the seriousness of AMS which accounts for 75% of helicopter interventions on Mont Blanc being the result of AMS and exhaustion.

(see also "Do you run course of less than 6 days?".