Heat stroke is a form of hyperthermia, an abnormally elevated body temperature with accompanying physical and neurological symptoms. Unlike heat cramps and heat exhaustion, two forms of hyperthermia that are less severe, heat stroke is a true medical emergency that can be fatal if not properly and promptly treated.

The body normally generates heat as a result of metabolism, and is usually able to dissipate the heat by either radiation of heat through the skin or by evaporation of sweat. However, in extreme heat, high humidity, or vigorous exertion under the sun, the body may not be able to dissipate the heat and the body temperature rises, sometimes up to 106°F (41.1°C) or higher. Another cause of heat stroke is dehydration. A dehydrated person may not be able to sweat fast enough to dissipate heat, which causes the body temperature to rise.

Those most susceptible to heart strokes include:

infants, the elderly (often with associated heart diseases, lung diseases, kidney diseases, or who are taking medications that make them vulnerable to heat strokes), athletes, and outdoor workers physically exerting themselves under the sun.