Feeling cold when you’re sick is your body’s way of generating heat to reach a new, higher internal temperature. When your immune system detects an infection, it essentially raises the thermostat in your brain. Your normal body temperature suddenly registers as “too cold” by comparison, so you experience chills, shivering, and an overwhelming urge to crawl under a blanket, even though your actual temperature may already be climbing above 37°C (98.6°F).
How Your Brain Resets the Thermostat
The process starts with your immune system, not your brain. When your body fights off a virus or bacterial infection, immune cells release signaling molecules called cytokines. Two of the most important ones cause the temperature-control center in your brain, a region called the hypothalamus, to raise its thermal set point. Think of it like someone turning the dial on your home thermostat from 20°C to 22°C: the furnace kicks on because the house is now “too cold” relative to the new target.
In your body, the hypothalamus coordinates a precise chain of events to close that gap between your current temperature and the new, higher set point. This is the moment you start feeling cold, even if a thermometer would already show a low-grade fever between 37.3°C and 38°C (99.1°F to 100.4°F). Everything your body does next is designed to produce and conserve heat.
Why You Feel Freezing: Blood Flow and Shivering
Your body has two main strategies for heating up quickly. The first is reducing heat loss. Blood vessels near the surface of your skin constrict, pulling warm blood away from your hands, feet, and skin and redirecting it toward your core organs. This is why your fingers might feel icy and your skin looks pale during the early phase of a fever. With less warm blood near the surface, your skin actually is colder to the touch, reinforcing the sensation that you’re freezing.
The second strategy is generating new heat. Shivering is the most obvious example. It’s an involuntary response where your skeletal muscles contract rapidly and repeatedly. These contractions are deliberately inefficient: they burn energy (ATP) in a way that produces heat as a byproduct rather than useful movement. Your body also reduces sweating during this phase, plugging another route through which heat would normally escape. Some heat generation happens without shivering too, through the activation of brown fat tissue, which burns calories specifically to produce warmth.
Together, vasoconstriction, shivering, and reduced sweating work in concert to push your core temperature up toward the new set point. Once your body reaches that target, the chills typically subside, even though you now have a fever. You stop feeling cold because the gap between your actual temperature and the set point has closed.
Why Fever Actually Helps You Fight Infection
This whole process exists for a reason. A mild fever slows down the invader and speeds up your immune response. Higher body temperatures reduce viral replication by interfering with the virus’s ability to enter cells and copy its genetic material. Temperatures above 41°C (105.8°F) can even destabilize the enzymes some viruses use to replicate, though fevers that high are rare and potentially dangerous on their own.
The benefit to your immune system is just as striking. Research from the University of Geneva found that warming immune cells from normal body temperature (37°C) to fever-range temperature (41°C) increased their migration speed by up to tenfold. White blood cells, including T cells, macrophages, and neutrophils, moved dramatically faster and entered lymphatic vessels in significantly greater numbers. The response was nearly instantaneous, occurring within seconds of the temperature change. The mechanism comes down to a motor protein inside immune cells that generates more mechanical force at higher temperatures, essentially giving your immune cells a turbo boost when you need it most.
Even antibiotics work better at elevated temperatures. One study testing 17 different antibiotics against 432 bacterial strains found a progressive increase in antimicrobial activity as temperatures climbed from 35°C to 41.5°C.
What Happens When the Fever Breaks
Once your immune system begins to gain the upper hand, cytokine production slows, and the hypothalamus resets its thermostat back toward normal. Now the opposite problem occurs: your body temperature is above the new (lower) set point, so the brain triggers cooling mechanisms. Blood vessels near the skin dilate, bringing warm blood to the surface. Sweating kicks in. This is the “fever breaking” moment, and it’s why you often wake up drenched in sweat after a night of chills.
This phase can cycle. Many infections cause fevers that rise and fall repeatedly over hours or days, meaning you might alternate between feeling freezing and feeling overheated as your immune system continues to fight.
Managing Chills Without Making Things Worse
The instinct to pile on blankets when you have chills is understandable, and it’s not necessarily wrong. Trying to cool yourself down during the chill phase tends to make you feel worse without offering much benefit. Using a light blanket or staying under covers is reasonable while your body is still working to reach its new set point.
The key is moderation. Burying yourself under heavy quilts can push your body temperature higher than the set point, trigger excessive sweating, and increase your risk of dehydration. One blanket is usually enough. Once the chills pass and you start feeling warm or sweating, that’s your cue to remove layers and let your body’s cooling mechanisms do their work. Staying well hydrated matters throughout the entire cycle, since both shivering and sweating burn through fluids faster than normal.
High-grade fevers above 39.1°C (102.4°F) deserve closer attention, particularly in young children and older adults. Temperatures above 41°C (105.8°F) move into dangerous territory where the fever itself can cause harm, though reaching that threshold from a common infection is uncommon.