Hyperthermia, hy puhr THUR mee uh, is a condition that occurs when the body becomes overheated. Normally, when the temperature of the body increases, excess heat is released through the skin. Blood vessels near the skin dilate (widen) to bring warm blood to the surface. Heat then escapes and the body cools. The body is also cooled by the evaporation of sweat. When the body is unable to release heat in these ways, illness occurs. Hyperthermia differs from hypothermia, a condition in which body temperature is below normal (see Hypothermia).