Altitude sickness - You need to understand them but not fearful. Fluid build up in your lungs or brain. They are relatively easy to detect. You can't breath for example. Or you have headache. Certain percentage of human population is susceptible to high altitude. Ivan Chouinard is one of them. If so, stay low. If you have symptoms like confusion or difficulty breathing, descend at least 300 meters (1000 feet). Repeat descent if your conditions do not improve.