Animals (including us) use a system called homeostasis to maintain a constant internal environment. This keeps temperature, pH and blood glucose level all constant.
There are a number of systems in place to keep this all level, these are called homeostatic systems. They detect a change and respond by negative feedback. The systems involve receptors (these detect the changes), a communication system (to communicate the changes) and effectors which make the change.
Hypothermia occurs when the body temperature is too low, this involves positive feedback. Below 35C and you're hypothermic. It happens when heats lost from the body quicker than it can be produced. As body temperature falls the brain doesn't work properly, shivering stops and the body temperature falls even more - basically, you're in trouble!
Positive feedback takes the body temperature further away from the normal level, body temperature continues to decrease unless some action is taken! - this system of positive feedback isn't involved in homeostasis because it doesn't keep your internal environment constant.
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