When climbing Kilimanjaro, the most frequent symptom of altitude sickness is shortness of breath. It occurs when your body is unable to absorb as much oxygen as it once did. Some climbers may notice it on their first day, similar to how they would feel short of breath when ascending stairs. Others may just notice that breathing becomes more difficult over 4,000 meters (13,000 ft). Everyone, though, will feel shortness of breath on the ascent at some point — while ascending, that is. It is rare that you will suffer shortness of breath when resting if you are in excellent health.