Fit, experienced walkers get altitude sickness while less athletic companions feel fine. Fitness is not the variable, and understanding what is changes how a high trip should be planned.
The problem is reduced oxygen pressure
Air at altitude has the same proportion of oxygen as at sea level, but lower pressure means fewer molecules in each breath. The body receives less oxygen per breath regardless of effort.
The response is a set of physiological adjustments including faster breathing, changes in blood chemistry and eventually increased red cell production. These take days rather than hours.
Fitness improves how efficiently the body uses oxygen it has. It does not change the rate at which those adaptations occur.
Rate of ascent is the dominant factor
Symptoms arise when a person arrives at an altitude faster than their physiology can adjust. The height reached matters less than how quickly it was reached.
Standard guidance emphasises limiting the gain in sleeping altitude each day above a threshold, and building in rest days. Climbing higher during the day and sleeping lower is a long-standing practice.
Arriving by air or road at a high destination removes the gradual gain entirely. That is why some of the worst cases occur on trips involving no walking at all.
Susceptibility is individual and inconsistent
Some people are affected reliably and others rarely, and the difference is not well predicted by age, sex or training. Previous experience at altitude is the best available indicator.
Even that is imperfect, since the same person can react differently on different trips. Hydration, recent illness and sleep all appear to contribute.
Because it is individual, group itineraries have to be planned around the slowest to acclimatise rather than the average.
Severity separates discomfort from emergency
Mild symptoms commonly include headache, poor appetite, disturbed sleep and fatigue. They typically settle with rest at the same altitude.
The serious forms involve fluid accumulation in the lungs or the brain, and they are medical emergencies. Confusion, loss of coordination and breathlessness at rest are the signs usually described.
The recognised response to worsening symptoms is descent, and delay is what makes cases dangerous. Anyone planning time at altitude should discuss it with a doctor beforehand.
Planning around it rather than through it
Itineraries with acclimatisation days built in cost time and remove the main risk factor. Compressed schedules are the most common cause of trouble on commercial trips.
Evacuation options and communications matter more at altitude than in most terrain, because the treatment is movement. Remote routes need that considered in advance.
Insurance conditions frequently reference altitude thresholds explicitly. Checking the policy against the route's high point is a practical step before booking.