Shivering with a fever happens because the brain has raised the body’s “set point” temperature to help fight infection, and the body is working to heat itself up to that new target.

Reset body thermostat

When germs trigger the immune system, chemical signals (pyrogens) tell the hypothalamus in the brain to set the body temperature higher than normal. Viruses and bacteria tend to grow less effectively at higher temperatures, so this rise in set point helps slow their growth and supports immune defenses.

Why you feel cold and shiver

Once the set point is raised, your actual body temperature is suddenly “too low” relative to the new target, so you feel cold even though a thermometer may show a fever. To close this gap, the body triggers shivering, which is rapid, involuntary muscle contractions that generate heat and raise core temperature toward the new set point.

Role of shivering in fever

The heat produced by shivering, plus behaviors like bundling in blankets when you feel chills, helps the body reach the higher temperature more quickly. Once the new set point is reached, the chills usually stop; later, when the set point is lowered again, you often sweat as the body sheds the extra heat.