Hypothermia occurs when the body loses heat faster than it can generate it, leading to a situation where the body core is unable to maintain sufficient warmth to allow the proper function of vital organs.
Wet, wind and cold are the key environmental factors that cause hypothermia. Any two of these is enough to start the cooling process in earnest.
As you move it is important to vent the moisture your body is creating as you exert yourself. At the same time you will attempt to protect yourself against the wind and rain. It is vital to keep your base layer as dry as possible, as wet or damp clothing against your body will cause you to lose heat 25 times faster than when dry.
A few degrees is all it takes!
Shivering occurs when core temperature 35C. The body is automatically trying to produce warmth by muscular activity.
Below 35C, we start to experience clumsiness, irrational behaviour and generally confused state. A person may appear to be drunk. At this stage the person suffering is almost exhausted and unable to produce body heat, let alone keep moving. Exercising the person will only increase the hypothermia. It would be dangerous to take the person suffering from hypothermia into a warm room as it can cause the blood to rush from the core to the extremities which suddenly may sent the person into a coma.
At 33C the person will be shivering, incoherent and complain of aching stiff muscles. They are now on the edge of severe hypothermia. If the temperature continues to drop they will slip into unconsciousness and may lose response to painful stimulus.
At 28C the person will give the appearance of death. It is important not to give up on treatment. There have been several cases where the person has made a full recovery having been kept alive by rescue breathing and CPR for several hours! For this reason a doctor will only pronounce the person dead when the body has returned to room temperature.