As altitude increases, the amount of oxygen in the air decreases, making it harder to breathe and requiring the body to work harder to get oxygen. The body responds by increasing red blood cell count to carry more oxygen and fluid can leak into the brain due to lower air pressure. Common high altitude illnesses include acute mountain sickness causing headache and nausea, and more severe illnesses like high altitude pulmonary edema and cerebral edema can cause fluid buildup in the lungs or brain and be life threatening without immediate descent to lower altitudes. The body needs time to acclimatize to higher altitudes to adapt to lower oxygen levels.