When illness strikes, the familiar feeling of being wrapped in blankets yet still shivering is a common experience. This sensation of coldness, often accompanied by chills, is actually a sign that your body is deliberately raising its internal temperature to fight off an infection. The chills are not a malfunction but a carefully orchestrated biological response, an attempt to reach a new, higher thermal setting. This controlled temperature increase, known as a fever, is one of the oldest protective strategies in the animal kingdom.
The Body’s Thermostat Reset
The body’s temperature is normally regulated by the hypothalamus, a small area in the brain that functions much like a central thermostat. This area maintains a specific thermal target, known as the set point, typically between 37°C and 38°C (98.6°F and 100.4°F).
When you become ill, substances known as pyrogens enter the bloodstream, acting as chemical messengers to signal an infection. These pyrogens originate either from invading microbes or are released by your own immune cells.
The pyrogens travel to the hypothalamus and trigger the production of Prostaglandin E2 (PGE2). PGE2 acts directly on the hypothalamic neurons, effectively “tricking” the brain into raising the temperature set point to a higher level. This upward shift creates a fever, as the body now perceives its current normal temperature as being too low.
Generating Heat: Why You Feel Cold
After the hypothalamus resets the temperature target, the body initiates a chain of physical reactions to rapidly close the gap between its current temperature and the new, elevated set point. The chills and sensation of cold are a direct result of these heat-generating and heat-conserving efforts.
One of the first physical responses is vasoconstriction, the narrowing of blood vessels close to the skin’s surface. This action shunts warm blood from the extremities toward the body’s core, significantly reducing heat loss. The reduced blood flow makes the skin feel cool to the touch and contributes to the feeling of coldness, even as the core temperature rises.
To actively generate the necessary heat, the hypothalamus triggers shivering, a series of rapid, involuntary muscle contractions. Shivering can dramatically increase the body’s heat production, sometimes by as much as four to five times the normal rate. Once the core temperature matches the new set point, the shivering ceases, and the sensation of cold subsides.
The Resolution: When the Fever Breaks
The fever phase concludes when the immune system overcomes the infection, causing the levels of pyrogens in the bloodstream to drop. With fewer pyrogenic signals, the hypothalamus resets the temperature set point back down to its normal, pre-illness range. This downward adjustment signals the “breaking” of the fever, requiring the body to rapidly cool itself to match the lower set point.
To dissipate the excess heat, the hypothalamus initiates two primary cooling mechanisms. The first is vasodilation, the widening of blood vessels near the skin. This allows warm blood to rush back toward the skin’s surface, where heat can be radiated away.
The second, and most effective, cooling response is sweating. The evaporation of sweat from the skin’s surface is a highly efficient way to remove large amounts of heat, leading to the characteristic clammy feeling often associated with the end of a fever.