Altitude sickness, also called acute mountain sickness (AMS), is a sickness that people can get when they climb quickly to high altitudes. It can also happen if they live at high altitudes but have not been there long. There are three types of altitude sickness: acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE). When people get AMS, their body does not get enough oxygen because the air pressure is lower than at sea level. This can cause headaches, nausea, vomiting, and death in severe cases.