Why a fever can make you feel freezing
A fever does not simply heat the body like a thermostat turned up from the outside. During many infections, chemical signals prompt the brain’s temperature-control center, the hypothalamus, to raise its temperature set point—the level the body tries to maintain. Your actual temperature may already be climbing, but compared with this newly raised target, you feel cold.
The body responds as it would to being chilled: blood vessels near the skin narrow to conserve heat, and muscles begin to contract rapidly. Those involuntary contractions are shivering. They use energy and release heat, helping bring body temperature closer to the brain’s new target.
What the shivering accomplishes
Shivering is therefore a heat-making response, not a separate remedy for infection. The fever itself is part of the body’s response to illness; a higher temperature can affect how some microbes and immune processes function. But shivering’s immediate job is narrower: to warm the body until it reaches the elevated set point.
That also explains why chills may arrive as a fever is rising. Once the set point comes back down—for example, as the fever eases—the body can feel too hot instead. It may then widen skin blood vessels and produce sweat to shed heat. The same temperature-control system that drove the shivering has switched from conserving heat to releasing it.
The striking detail is that someone can be hot to the touch and still shiver intensely: the brain is comparing body temperature not with its usual target, but with the temporarily higher one.
