Why do we get a fever when we are ill?

When the immune system detects an infection, white blood cells release chemicals called pyrogens that signal the hypothalamus, the brain’s thermostat, to raise the body’s set-point temperature. The body then shivers, constricts skin blood vessels and increases metabolism to generate heat. Higher temperatures slow the reproduction of many pathogens and increase the speed and effectiveness of immune cells. Most fevers in adults resolve on their own as the infection is cleared. Very high fevers, above about 40 °C, can be dangerous and may need medical attention.