Hypothermiais a serious medical condition where body temperature falls to levels which seriously slows or even prohibits metabolism. Prolonged and/or extreme hypothermia can be fatal. A common treatment is to immerse a patient is a warm bath to their bring body temperature up as quickly as possible.
In 1995, Agent Mulder developed extreme hypothermia after prolonged exposure to the cold Alaskan tundra. After being rescued he was transferred to a medical facilty where physicians employed the standard immersion treatment. This however would have led to the re-animation of a lethal alien retro-virus; the treatment was stopped at the request of Agent Scully (TXF: "Colony").