Hypothermia is when the core body temperature drops to a level where normal brain and muscle function is impaired - usually at or below 35°C. When the body cannot cope, it goes into survival mode, shutting down non-essential functions. Hypothermia occurs when the body cannot make up for the amount of heat lost.
7% of outdoor recreation fatalities were caused by hypothermia– There and Back, 2016
Hypothermia can be avoided by wearing appropriate clothing and ensuring you have sufficient food and shelter.