Fever is usually described as something an infection does to the body. The physiology points the other way: the body raises its own temperature deliberately, following a set point the brain has moved.
The body defends a temperature target
A region of the hypothalamus behaves like a thermostat, comparing incoming temperature signals against a target and triggering corrections. Cooling responses include sweating and dilating vessels near the skin.
Warming responses include narrowing those vessels, raising muscle tone and shivering. These run constantly in the background, which is why core temperature stays within a narrow band across very different environments.
Fever is not a failure of that system. It is the same system working normally toward a target that has been shifted upward.
Immune signaling moves the set point
When immune cells encounter certain molecules from bacteria, viruses or damaged tissue, they release signaling proteins that circulate and act on the temperature-regulating region.
Those signals raise the target the hypothalamus is defending. Nothing has yet heated the body; only the number the brain is aiming for has changed.
The distinction matters because it explains the sequence people experience. The brain now judges a normal temperature to be too cold and begins correcting it.
Chills come first because the body is behind its target
A person entering a fever often feels cold in a warm room, pulls on blankets and shivers. The room has not changed; the reference temperature has.
Shivering is muscular work that generates heat, and vessel constriction keeps that heat from escaping through the skin. Together they push core temperature up toward the new target.
When the fever later breaks, the set point drops back and the same machinery runs in reverse. Sweating and flushing appear because the body is now above target.
Height and course carry different information
A temperature reading describes where the body currently sits relative to its target, not how dangerous the underlying cause is. Serious infections sometimes produce modest readings, and mild ones sometimes produce high ones.
Clinicians pay attention to the pattern over time, other findings, and the person's age and medical history. In infants, older adults and people with weakened immune function, the usual relationships shift considerably.
Some conditions also raise body temperature without moving the set point at all, which is a different mechanism with different implications.
Where measurement stops and judgment begins
Home thermometers differ by site and method, and oral, ear, forehead and rectal readings are not interchangeable. Comparing readings taken different ways produces confusion rather than information.
Deciding what a fever means in a particular person requires a clinician who can weigh the full picture, and anyone worried about a fever should seek that assessment directly.
Understanding the mechanism does not answer the clinical question, but it does explain why the experience of chills and sweats follows the pattern it does.