A Tibetan villager plays pool in the outskirts of Basu County of Tibet Autonomous Region, China. The Tibetans are accustomed to the high altitude of their region, though many visitors are afflicted with high altitude sickness.

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Acute Mountain Sickness

Acute Mountain Sickness (AMS) is the most common manifestation of altitude sickness. AMS doesn't normally occur below 8,000 feet (2,438.4 m)

-- but you don't have to climb a mountain to feel its effects. In the United States, you might get symptoms of AMS in Vail, which sits at a base of 8,120 feet (2,474.98 m). In the Nepalese valleys near Mount Everest, elevation ranges from lower altitudes around 6,500 to 10,000 feet (1,981.2 to 3,048 m) to higher altitudes of 11,000 and 13,000 feet (3,357.8 and 3,962.4 m). La Paz, Bolivia, sits at 11,975 feet (3,649.8 m). Visitors to these sites -- even those who arrive by ways other than foot -- can feel the effects of altitude sickness.

AMS is an altitude-related neurological disorder, so it's nothing to take lightly. Though the causes of AMS are still not entirely understood, genetics seem to play a part. An out-of-shape person may not be vulnerable to AMS, while an Olympic athlete may find himself or herself in a life-or-death medical emergency. People with AMS sometimes attribute headache or nausea to some other cause and continue ascending -- and getting sicker. This misdiagnosis can be fatal.

There are three forms of AMS: mild, moderate and severe.

Most people who ascend to altitudes of 10,000 feet (3,048 m) or more will have at least some symptoms of mild AMS. These include headache, nausea, weakness and decreased appetite. The onset of mild AMS generally occurs within 12 to 24 hours of reaching a higher altitude and will normally disappear within three days. While there are medications that can treat the symptoms of AMS, they can't cure it. The only way to cure AMS is resting and allowing your body to acclimatize -- adjust to the change in altitude -- or descending to a lower altitude. Most people afflicted with AMS are ambulatory; only in severe cases will they need to be carried or otherwise transported down.

If your symptoms become worse, you may have progressed to moderate AMS. One alarming symptom of moderate AMS is ataxia, the loss of coordination. The hallmark of ataxia is the "drunk man's walk," or the inability to walk in a straight line.

If you suspect that you have moderate AMS, you should immediately descend. Acclimatization and medications won't help you at this stage. Even a few hundred feet of descent should quell symptoms, but the lower, the better. In one to three days, the symptoms may disappear entirely. At this point, you can return to a higher elevation -- but you must properly acclimate along your ascent.

If you or a companion has a severe loss of coordination, trouble thinking clearly and fluid buildup in the lungs, these are all signs of severe AMS. The afflicted party should immediately be taken to low altitude, preferably under 4,000 feet (1,219.2 m). Severe AMS can be deadly. But even severe AMS isn't as dangerous as the next condition we'll learn about.