Hyperthermia
Heat stroke happens when a person gets so hot that their body cannot do anything to bring their temperature down.  The body has tried every strategy it has to cool itself down.  But the body is so hot that none of those strategies work any more.  This causes the body temperature to rise very quickly.  The body gets so hot inside that its tissues, especially the brain, get damaged.  Usually, people with heat stroke have a body temperature of 104 degrees Fahrenheit or higher.  The brain cannot survive for long at these temperatures.  Heat stroke can very quickly cause damage to the brain, heart, kidneys, and muscles.