For most people, it causes mild symptoms that improve with rest and with time spent getting used to altitude. However, in some people, it can lead to serious symptoms which can become life-threatening. This is particularly the case if the signs are not recognised and the person does not descend to a lower altitude.
This is a companion discussion topic for the original article at https://patient.info/travel-and-vaccinations/health-advice-for-travel-abroad/altitude-sickness