The technical term for
altitude sickness.
If you ascend to a higher altitude faster than your body can compensate, you may suffer from AMS. Your lungs do not function as well, due to the lower concentration of oxygen in the air, and the thinner overall density and pressure of the ambient air. This causes headache, nausea, shortness of breath, dizziness, drowsiness, and fatigue.
If left untreated (you keep ascending, or fail to descend) it can develop into pulmonary edema (HAPE) or cerebral edema, which are very serious and life-threatening.
The cure is to survive long enough to acclimate to the altitude or sit in a Gamow Bag for a while. There are other things you can do to help avoid it, though. Drink plenty of water. Stay away from caffeine and alcohol. Your doctor may give you a prescription for Diamox. People who climb Everest and K2 often bring liquid oxygen bottles with them.
The treatments do not match the cure, however. If you begin to feel symptoms, get to a lower altitude.