Heatstroke is a serious condition that occurs when the body is unable to control its temperature due to excessive heat. Body temperature rises sharply, causing hyperthermia (body temperature greatly above normal) and dehydration (loss of water from the body).

In normal circumstances, the body controls temperature by disposing of heat through the skin, mainly by sweating. The normal temperature is 36-36.8C (96.8-98.24F). However, very high temperatures of over 40C (104F) can cause the body's internal thermometer to fail so that heat is not disposed of adequately and the body cannot cool down in the usual way.