High Altitude Pulmonary Edema. An often fatal form of altitude sickness (though not the same as AMS) in which the alveoli (of the lungs) fill with fluid, impairing gas exchange. HAPE has killed seasoned Himalayan climbers, casual hiking tourists, people who've grown up at sea level, and people who've grown up in Namche Bazaar at 11,000 feet. HAPE occurs more often in pre-adolescents compared to adults. Low altitude residents also suffer increased risk of HAPE when exerting themselves at unaccustomed high altitudes. Symptoms include disorientation, elevated blood pressure and heartrate, shortness of breath (dyspnea at rest), cough, chest tightness, or crackling/gurgling in at least one lung. More severe symptoms follow if left untreated. Commercial prescription medication designed to counter HAPE includes Diamox, and many climbers have reported positive results using this drug. The condition's onset is often at night.
IANAD.
Good ways to prevent HAPE (and other altitude disorders) include gradual acclimatization and altitude increases, travelling with experienced and attentive companions, proper hydration and nutrition when possible, supplemental oxygen at high altitudes, and certain medications. Please, please consult knowledgable sources (such as the Himalayan Rescue Association, HRA, www.gorge.net/hra) if you intend to undertake even casual, moderate altitude hiking. Effective treatments include rapid descent, oxygen, water, and possibly the use of a Gamow bag if necessary, though as far as I know that has not been proven 100% effective at fooling the body into believing it's really at a lower altitude. Portable hand-held pulse oximeters can be used to detect this disorder.