Negative feedback is activated by the body when something such as body temperature, pH of the blood, sugar concentration in the blood, gas concentrations, and water balance are not within very strict boundaries. It is a part of homeostasis.
The diagram below shows how negative feedback helps keep the blood temperature between 35.8 and 37.8 degrees celcius , which are the limits the human body can take.
As can be seen, a stimulus is needed to cause a drop or incease in blood temperature. A temparature that is too high will activate the heat-loss center in the hypothalamus, which causes blood vessels to dilate (so that heat can travel in the blood stream), and activates sweating (because the evaporation of sweat causes body temperature to decrease).
In case the temperature is too low, skin blood vessels will contract (to keep the blood from losing heat), and shivering begins (the contraction of the muscle that occurs when one shivers causes heat).