When you are sick, the infectious agent causes the body to react, releasing substances that cause fever. Via the bloodstream, they reach a small area of the brain: the hypothalamus.
Responsible for thermoregulation, this area of the central nervous system is normally responsible for maintaining body temperature at around 37°C. But in this case, the thermoregulation system causes the temperature to rise.
> On the one hand, it limits the body's heat loss by reducing blood circulation on the surface of the skin. This phenomenon of vasoconstriction makes it possible to minimise the amount of heat transported by the blood from the deep organs to the surface of the skin. The skin then cools.
> On the other hand, it increases heat production by creating shivers. Muscle tone increases, as muscles contract involuntarily releasing muscle energy that contributes to increasing body temperature.
The shivers are often accompanied by goose bumps, also known as "horripilation". It is due to the presence of small muscles at the base of the hair. As they contract, they straighten the hair, creating small bumps on the surface of the skin.