Altitude Sickness might occur when you go up a mountain too fast and cannot get acclimated to the altitude. It usually happens at 2500 m or higher altitude. Physical signs and symptoms may include a headache, loss of appetite, and trouble sleeping. Most people would recover by themselves when they get used to the height. However, sometimes there is an exception.
For more information or prevention of the altitude sickness, please check the link: Altitude Sickness