The high-altitude (HA) environment generally refers to elevations over 1500 m (4800 feet) above sea level. High-altitude illnesses is the term given collectively to Acute Mountain Sickness (AMS), High-Altitude Cerebral Edema (HACE) and High-Altitude Pulmonary Edema (HAPE), the latter two being potentially fatal conditions. The complete pathophysiological mechanisms of these maladies are not completely known and their incidences are highly variable amongst different ethnic populations. Some individuals are able to acclimatize better and are genetically more resistant to HA conditions compare to others. The occurrence and severity of the disease is highly dependent on the rate of ascent, the altitude attained and individual susceptibility.