What do you do for altitude sickness?

Safe descent. The best treatment for altitude illness or altitude sickness is to decrease one's elevation by at least 1000 ft (quickly and safely.) if symptoms do not improve, one needs to descend another 1000 ft. If one has symptoms of hace (high altitude cerebral edema) or hape (high altitude pulmonary edema) contact search and rescue and arrange for emergent air evacuation.
Depends. As above, high-altitude pulmonary edema is a potentially lethal illness with typical symptoms of wet cough and shortness of breath at rest. High-altitude cerebral edema is another potentially lethal illness with severe headache, confusion, staggering, vomiting. People can die. Mountain sickness is milder and can be treated but not without training. We work in it every day and still worry. Be safe.