The human body can regulate its core temperature to between 98 and 100 degrees F. If skin temperature drops below 37 degrees C. the body attempts to conserve heat by constricting the blood supply to the body surface. Shivering is the bodys attempt to produce more heat through muscle activity. Because no external work is performed during shivering, all the energy released is in the form of heat. Here is a short review on body temperature regulation. Also here.
Hypothermia (stage 1) occurs when the body temperature reaches 95 degrees or lower. Read about hypothermia at this Coast Guard website.
Hyperthermia occurs when the body produces more heat than it can dissipate. Body temperatures above 104 degrees F are life threatening. You can listen to scientist talk about how the human body copes with heat at this NPR website.
Fever (pyrexia)is an increase in body temperature as a response to bacterial or viral infection or to tissue injury. It's a temporary elevation in the bodies thermoregulatory setpoint. Generally, fever is not an illness because it's a defense response against infections. Body temperature can also increase due to hard exercise; ovulation; excess thyroid secretions and central nervous system damage. Cancer can also induce fever. More about fevers here.
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