Acute Mountain Sickness (AMS) is a constellation of symptoms that represents your body not being acclimatized to it's current altitude. As you ascend, your body acclimatizes to the decreasing oxygen (hypoxia). At any moment, there is an "ideal" altitude where your body is in balance; most likely this is the last elevation at which you slept. Extending above this is an indefinite gray zone where your body can tolerate the lower oxygen levels, but to which you are not quite acclimatized. If you get above the upper limit of this zone, there is not enough oxygen for your body to function properly, and symptoms of hypoxic distress occur - this is AMS.
Go too high above what you are prepared for, and you get sick. This "zone of tolerance" moves up with you as you acclimatize. Each day, as you ascend, you are acclimatizing to a higher elevation, and thus your zone of tolerance extends that much higher up the mountain. The trick is to limit your daily upward travel to stay within that tolerance zone.
The exact mechanisms of AMS are not completely understood, but the symptoms are thought to be due to mild swelling of brain tissue in response to the hypoxic stress. If this swelling progresses far enough, significant brain dysfunction occurs (See next section, on HACE). This brain tissue distress causes a number of symptoms; universally present is a headache, along with a variety of other symptoms.
The diagnosis of AMS is made when a headache, with any one or more of the following symptoms is present after a recent ascent above 2500 meters (8000 feet).
All of these symptoms may vary from mild to severe. A scoring system has been developed based on the Lake Louise criteria; look at the AMS questionnaire for a simple method to evaluate an individual's AMS severity.
AMS has been likened to a bad hangover, or worse. However, because the symptoms of mild AMS can be somewhat vague, a useful rule-of-thumb is: if you feel unwell at altitude, it is altitude sickness unless there is another obvious explanation (such as diarrhea).
Anyone who goes to altitude can get AMS. It is primarily related to individual physiology (genetics) and the rate of ascent; there is no significant effect of age, gender, physical fitness, or previous altitude experience. Some people acclimatize quickly, and can ascend rapidly; others acclimatize slowly and have trouble staying well even on a slow ascent. There are factors that we don't understand; the same person may get AMS on one trip and not another despite an identical ascent itinerary. Unfortunately, no way has been found to predict who is likely to get sick at altitude.
It is remarkable how many people mistakenly believe that a headache at altitude is "normal"; it is not. Denial is also common - be willing to admit that you have altitude illness, that's the first step to staying out of trouble.
It is OK to get altitude illness, it can happen to anyone. It is not OK to die from it. With the information in this tutorial, you should be able to avoid the severe, life-threatening forms of altitude illness.