High-altitude sickness, also known as acute mountain sickness (AMS), occurs when individuals ascend to high elevations too quickly. At higher altitudes, the air pressure decreases, leading to lower oxygen levels. Symptoms can include headaches, nausea, dizziness, and fatigue, typically appearing above 8,000 feet (about 2,400 meters). To prevent high-altitude sickness, it is essential to acclimatize gradually by ascending slowly and allowing the body time to adjust. Staying hydrated and avoiding alcohol can also help. In severe cases, descending to lower altitudes is necessary, and medical attention may be required for serious symptoms.