Medicine. Body overheating. Heat exhaustion can happen when the body loses too much fluid (due to excessive sweating) or when conditions, such as physical activity in a hot environment, prevent sweat from evaporating into the air. Heat Stroke is a sub classification of heat exhaustion. This is a potentially deadly condition in which over-exposure to a very hot environment breaks down the body's ability to control its temperature and cool itself sufficiently. The body temperature rises to a very high (deadly) level.