A fever is defined as an elevated body temperature, typically above the average of 98.6°F (37°C). Chills are the uncomfortable sensation of being cold, often accompanied by involuntary shivering. Feeling intensely cold when the body is objectively hot presents a paradox explained by a temporary shift in internal temperature regulation. This sensation is a direct result of the immune system actively raising the internal temperature to combat an infection. The feeling of coldness is a physiological tool the body uses to reach its new, elevated thermal goal.
Setting the Thermostat Higher
The body’s temperature is precisely managed by the hypothalamus, a small region in the brain that acts as the central thermostat. Under normal conditions, the hypothalamus maintains a steady temperature set point. When an infection occurs, the immune system releases specific molecules known as pyrogens (external substances from a pathogen or internal chemicals like cytokines). These pyrogens travel to the hypothalamus, triggering the production of Prostaglandin E2 (PGE2).
The presence of PGE2 effectively “resets” the hypothalamic set point to a higher temperature, similar to turning up a home thermostat. The body’s current temperature is then perceived as being too low relative to this new, elevated setting. The internal regulatory system interprets this state as hypothermia, initiating immediate responses to close the temperature gap.
Generating Heat: Why You Feel Cold
The feeling of coldness is a direct consequence of the body attempting to rapidly increase its core temperature to match the new set point. The body employs two primary physical mechanisms to generate and conserve heat. The first is vasoconstriction, where small blood vessels near the skin’s surface narrow significantly. This action reduces blood flow to the extremities and skin, minimizing heat loss and retaining warmth in the core.
The reduction in surface blood flow causes the skin to feel cool or appear pale, despite the internal temperature rising. The second mechanism is shivering, which involves rapid, involuntary contractions of the skeletal muscles. These muscle movements are highly metabolically active and generate a substantial amount of heat. The sensation of chills is the conscious experience of the body’s internal command to shiver and produce heat.
Shivering continues until the body’s core temperature successfully reaches the new set point. Once the temperature matches the new target, the sensation of cold and the shivering typically subside. The body then enters the plateau phase of the fever, where heat production and heat loss are balanced at the higher temperature level.
The Purpose and Phases of Fever
Fever is a coordinated defense mechanism that plays a beneficial role in fighting off infections. The elevated temperature enhances the effectiveness of immune cells and processes. A higher core temperature can speed up the maturation and migration of white blood cells involved in the immune response.
The elevated thermal environment also directly inhibits the replication rate of many types of bacteria and viruses. This biological slowdown gives the immune system a temporary advantage in clearing the infection. The fever process concludes in a phase known as defervescence, or the “fever breaking,” when the level of pyrogens begins to decrease.
The reduced pyrogen concentration signals the hypothalamus to return the temperature set point to its normal range. Since the body is now warmer than the thermostat’s setting, it initiates cooling mechanisms to shed the excess heat. This cooling is accomplished through vasodilation (increasing blood flow to the skin) and sweating (using evaporation to rapidly lower the temperature).
When Chills and Fever Require Attention
While fever is generally a sign of a working immune system, certain symptoms or temperature levels warrant medical evaluation. An extremely high temperature, such as 103°F or 104°F in adults, should prompt contact with a healthcare provider. A fever that persists for more than 72 hours, even if not extremely high, also requires a medical assessment.
Specific accompanying symptoms signal a need for immediate attention, regardless of the temperature reading. These warning signs include:
- Confusion or altered mental status.
- Severe headache.
- Neck stiffness.
- Difficulty breathing.
Chills that are violent (rigors) or those that do not subside can also indicate a more serious infection.
For comfort, over-the-counter medications like acetaminophen or ibuprofen are commonly used to temporarily lower the hypothalamic set point. If a fever occurs in an infant younger than three months of age, or if a person has a compromised immune system, medical consultation should be sought immediately. Focusing on rest and fluid intake remains a foundational practice for managing a fever at home.