Feeling cold and shivering when sick, even with a fever, is a common physiological response. It’s your body actively working to raise its internal temperature to a new, higher set point to combat infection.
The Body’s Internal Thermostat
The human body maintains a remarkably stable internal temperature, typically around 37°C (98.6°F), through a process called thermoregulation. This precise control is orchestrated by a small but powerful region in the brain known as the hypothalamus, which functions much like a thermostat. The hypothalamus continuously receives information from temperature sensors located throughout the body and adjusts heat production and loss to keep the core temperature within a narrow, healthy range.
To maintain this balance, the body employs various mechanisms. Heat is constantly generated through metabolic processes, and it is lost through mechanisms such as sweating, where evaporating moisture cools the skin, and vasodilation, which increases blood flow to the skin’s surface, allowing heat to radiate away. When the body needs to warm up, it can reduce heat loss by narrowing blood vessels in the skin (vasoconstriction) and generate heat through muscle activity like shivering.
The Immune System’s Role in Temperature Changes
When an infection occurs, the immune system plays a central role in triggering changes to the body’s temperature. Immune cells, particularly macrophages, detect invading pathogens like bacteria or viruses. In response, these cells release signaling molecules called cytokines, which act as endogenous pyrogens.
These cytokines, including interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α), travel through the bloodstream to the hypothalamus. Upon reaching the hypothalamus, these pyrogens induce prostaglandin E2 (PGE2), which “resets” the body’s thermostatic set point to a higher temperature. This initiates the fever response as a defense against infection.
Why Your Body Feels Cold While Warming Up
After the hypothalamus resets the temperature set point higher, the body perceives its current temperature as too low. This triggers responses to increase body temperature to the new set point. The feeling of coldness and chills result from these heat-generating and heat-conserving efforts.
To raise the body’s temperature, two primary mechanisms are activated. First, shivering begins, involving involuntary muscle contractions that rapidly generate heat. Second, vasoconstriction occurs, where blood vessels in the skin constrict to reduce blood flow to the body’s surface, minimizing heat loss to the environment. These actions collectively make a person feel cold and shiver even as their core body temperature is actually rising.
Additional Factors Influencing Body Temperature Sensation
Beyond the primary fever response, several other factors can influence how cold a sick individual feels. Dehydration, common during illness, can impair temperature regulation. Inadequate fluid levels make temperature fluctuations more noticeable and contribute to feeling cold.
Fatigue and reduced activity also play a role. Sickness decreases physical activity, lowering metabolic heat production. The body redirects energy to fighting infection, leaving less for warmth. This reduced heat generation can make a person more sensitive to their surroundings and worsen the feeling of cold.