First of all altitude sickness (or Acute Mountain Sickness aka AMS) generally starts to affect climbers at or above elevations around 6,500 ft, so if you are not planning on climbing higher than that, the chances or you developing AMS is less. Altitude sickness is a term that refers to the pathologic changes that occur within the human body due to the low partial pressure of oxygen at high altitude. It is an entity that is well known, but not studied exhaustively. For instance, we don't yet have a very effective way of predicting who will, or will not, develop AMS while climbing in high altitudes. While the percent composition of oxygen within the air does not change with altitude (21%), the actual density of the air does change, and this is what causes the physiologic effects seen. The symptom picture seen in altitude sickness is non-specific, but includes: nausea, anorexia (lack of appetite), fatigue, weakness, malaise (generalized feeling "sick"), epistaxis (nose bleeds), dizziness
, light-headedness, diarrhea
, and peripheral edema (swelling in the extremities).
If left untreated and the disease process progresses, it can cause more serious problems such as High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE) both of which can potentially fatal. This is why climbers should take the early signs/symptoms seriously. Ascending slowly and acclimating at different elevations is the best way to prevent the development of altitude sickness. If you have more specific questions, I would recommend seeing a physician that specializes in high altitude medicine. Hope this helps. Best of luck.