Hyperthermia

Hyperthermia is an elevated body temperature caused from external influences, generally a combination of one or more of dehydration, intense physical activity, high external temperatures, and heavy clothing for the conditions. It's common among athletes during summer months and those who work in warm environments, such as foundries. It's a common cause of death during heatwaves, particularly among the elderly, who are less capable of regulating their body temperature through sweating.
Humans regulate their body temperature through a variety of mechanisms to keep it within a narrow range. When the core body temperature rises due to muscular exertion or a warm environment, the body starts to sweat. Evaporation of sweat can remove heat even in a very warm environment. However, if sweat cannot evaporate due to the environment being too humid to absorb water, or the individual wearing too much clothing, the body has very limited ways of getting rid of excess heat (primarily through respiration). The body temperature continues to rise, resulting in rapid dehydration (thus the lack of sweat and saliva even in a very warm environment), delirium and confusion, followed by unconsciousness as the body ceases all activity in an attempt to prevent further buildup of heat.
Hyperthermia is very serious and immediate medical attention is called for. Dousing with cool water is recommended, although ice packs may in fact cause body heat to continue to rise in the core of the body. Medical personnel will usually treat with intravenous fluid, but rapid cooling with alcohol is used as well as monitoring core body temperature, usually through the anus.
See also: Heat stroke