HYPOTHERMIA, or low body temperature, is the result of overexposure to cold water or air.  This often seems to be a favorite choice of fic authors to use as a plot device to get two of the characters naked and wrapped up in a sleeping bag together.  It's also a convenient way to mildly incapacitate a character.

Mild Hypothermia is defined when the body temperature drops to 95° to 98°F.  (35° to 36.6° C).  Normal body temperature is 98.6° F (37°C).   The affected individual's skin is cold to the touch; he or she shivers and will start to lose skeletal muscle coordination.

Moderate Hypothermia results when the body temperature drops to 89.6° to 95° F (32° to 35° C).  The individual will be very cold, develop severe shivering, have slurred speech, and begin to have signs of a decreased level of consciousness and some amnesia.

Marked Hypothermia is when the body temperature drops to 82.4° to 89.6° F (28° to 32° C).  Muscle rigidity develops, the extremities start to turn blue (peripheral cyanosis) and signs of shock develop.  Consciousness diminishes and the person becomes very sleepy and difficult to arouse.

And finally, with Severe Hypothermia, the body temperature is 77° to 82.4° F (25° to 28° C).  The victim is unresponsive, deep tendon reflexes are absent and ventricular fibrillation may occur.  The person looks dead with no palpable pulse, audible heart beat or reflexes.  Body temperature that drops to less than 77° F (25° C) is incompatible with life and will result in cardiopulmonary arrest and death. 

Mild to moderate hypothermia can (theoretically) be treated with non-invasive means.  This means getting the victim warm and dry.  Move the victim to some place warm (but avoid too strenuous of activity), remove all wet clothing and keep him or her dry.  If the victim is conscious, give him or her something warm to drink with a high sugar content.  Hot water bottles can be used in the groin and under the arms. This is the level of hypothermia where you can strip your patient and rescuer down to their undergarments (or beyond) to help restore body temperature.

Marked and severe hypothermia require much more aggressive treatment provided by trained medical personnel.  Warm intravenous solutions; warm humidified oxygen to breath, warming blankets and the like.  All of these interventions require medical equipment that wouldn't be available in a field situation.  You need to get your patient to a medical facility quickly, to realistically ensure his or her survival. 

In severe hypothermia, where the victim appears dead, CPR should be instituted and continued for two to three hours or until the victim's core body temperature is returned to normal.  The lower body temperature can preserve brain function longer, so resuscitation is not stopped until the patients core temperature is normal.  In extreme cases, peritoneal lavage with warm fluids or cardio-pulmonary bypass to directly warm the patient's blood can be considered.  In a cardiac arrest situation, defibrillation is not even considered until core body temperature is at least 80° F (26.6° C) as the heart's conduction system simply cannot work at temperatures colder than that.

For more information: http://www.hypothermia.org/hypothermia1.htm and http://www.hypothermia.org/hypothermia3.htm

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At the other end of the temperature spectrum are HEAT CRAMPS, HEAT EXHAUSTION and HEAT STROKE. 

Heat cramps are a minor problem; cramps of the calf muscles caused by dehydration, high temperatures and lack of physical conditioning.  While painful, they're not life threatening and respond well to fluids, cooler temperatures and rest.

Heat exhaustion is a much more serious condition of extreme weakness and fatigue caused by high temperatures and dehydration.  Body temperature can rise to over 102° F (38.8° C). Symptoms of heat exhaustion can include muscle cramps, pallor, nausea and vomiting, extreme fatigue, dizziness, lightheadedness, rapid thready pulse, headache, confusion, fainting, thirst and cool, clammy skin.  The individual suffering from heat exhaustion should be protected from the heat and placed in a shady place, given liquids to drink (i.e. sports drinks are good), salt tablets and cold, wet compresses can be used.  Loosen clothing and massage muscles if cramping has occurred.  Hospitalization is usually not required, though intravenous fluids can be beneficial.

Heat stroke is a medical emergency and requires hospitalization.  Symptoms include severe headache and muscle cramps (which may disappear); vomiting; high blood pressure followed by a drop in blood pressure; rapid pulse; hot, dry, red skin which later turns grayish in color and no sweating; confusion and then loss of consciousness; fever greater than 102.2 ° F (39° C); dilated pupils; slow, deep respiration's with pauses; body odor of burned flesh. 

Treatment is aimed at replacing fluids and electrolytes with intravenous fluids, maintenance of respiratory and cardiac function, lowering of body temperature by the use of hypothermia blankets and ice packs, the use of muscle relaxants intravenously to reduce shivering and steroids to reduce edema. 

Avoiding heat exhaustion and stroke is the best treatment.  In hot weather, one should wear loose-fitting, lightweight clothing (they were desert camouflage for a reason, light colored clothing reflects heat, dark colors absorb heat), rest frequently and drink plenty of electrolyte-containing liquids.

For more information: http://www.cdc.gov/nceh/hsb/extremeheat/heatstroke.htm