Chills during illness are your body’s way of generating heat to reach a higher internal temperature. When you’re fighting an infection, your brain essentially resets its internal thermostat to a higher target, and until your body catches up to that new target, you feel cold and start shivering. It’s the same sensation you’d feel standing outside on a freezing day, except the “cold” is manufactured entirely by your own immune system.
How Your Brain Resets Its Thermostat
Your brain has a temperature control center that normally keeps your core body temperature hovering around 98.6°F (37°C). When bacteria, viruses, or other pathogens invade, your immune cells release signaling molecules called cytokines. These molecules travel through your bloodstream and reach blood vessel cells in your brain, which respond by producing a chemical messenger called prostaglandin E2.
Prostaglandin E2 is the final trigger for fever. It acts on a specific group of neurons in the brain’s temperature control center, essentially silencing them. Under normal conditions, these neurons actively keep your body temperature from rising by sending out inhibitory signals. When prostaglandin E2 switches them off, the brakes come off, and your brain sets a new, higher target temperature. Instead of 98.6°F, your body might now be aiming for 101°F or 102°F.
This is why chills happen at the beginning of a fever, not at the peak. Your actual body temperature is still normal, but your brain thinks it should be several degrees higher. The gap between where you are and where your brain wants you to be is what makes you feel freezing cold.
Why Shivering Produces So Much Heat
Once your brain decides you’re “too cold,” it activates the same warming responses it would use if you were actually exposed to frigid weather. Blood vessels near your skin constrict, pulling warm blood away from the surface to reduce heat loss. That’s why your skin may look pale or feel cool to the touch. Your tiny skin muscles contract, producing goosebumps, a leftover reflex from when our ancestors had enough body hair for it to provide insulation.
The most powerful response is shivering. Multiple large muscle groups throughout your body begin contracting rapidly and involuntarily, each at relatively low intensity (less than 20% of their maximum force). But because so many muscles are recruited simultaneously, the combined effect is enormous. Shivering can increase your metabolic rate four to five times above resting levels, with some individuals reaching six or seven times their baseline. All of that extra metabolic activity converts energy directly into heat, driving your core temperature upward toward the brain’s new target.
Once your temperature reaches the new set point, the shivering stops, and you enter the plateau phase of fever. You’ll feel warm, maybe uncomfortably hot, but the chills are gone. Later, when your immune system gains the upper hand and prostaglandin E2 production drops, the thermostat resets back to normal. Now your actual temperature is above the target, so your body flips to cooling mode: blood vessels dilate, you start sweating, and the fever “breaks.”
Why Your Body Bothers With Fever at All
Fever isn’t a malfunction. It’s a deliberate immune strategy that has been conserved across vertebrate species for hundreds of millions of years, which suggests it provides a real survival advantage. Higher body temperatures slow the replication of many bacteria and viruses, which tend to thrive at normal human body temperature. At the same time, elevated temperatures speed up certain immune responses, helping your white blood cells move faster and work more efficiently.
The trade-off is significant energy expenditure. Raising your body temperature by just a couple of degrees requires a substantial increase in calorie burn, which is one reason you feel so drained during a fever. Your body is essentially running a furnace while simultaneously fighting off invaders.
Chills vs. Rigors
Not all chills are equal. Mild chills involve a general feeling of coldness with light shivering. Rigors are more intense: visible, uncontrollable shaking that can last several minutes and leave you exhausted. Research has found that these severe shaking chills are a more specific predictor of a bacterial infection in the bloodstream (bacteremia) compared to mild chills alone. If you experience violent, teeth-chattering shaking rather than just feeling cold, it may signal a more serious infection.
How Fever Reducers Stop the Chills
Over-the-counter fever reducers like acetaminophen, ibuprofen, and aspirin work by blocking the production of prostaglandin E2 in the brain. They inhibit the enzyme (cyclooxygenase) that your brain’s blood vessel cells need to manufacture prostaglandin E2 in the first place. With less prostaglandin E2 acting on the thermostat neurons, the set point drops back toward normal. Your body then recognizes it’s “too warm,” stops shivering, opens up blood vessels near the skin, and begins sweating to shed the excess heat.
This is why taking a fever reducer both stops the chills and eventually brings your temperature down. It’s addressing the root cause: the brain’s artificially elevated target. Bundling up in blankets may help you feel warmer during the chills phase, but it doesn’t change the set point itself.
When Chills Signal Something Serious
Most chills that accompany a common cold, flu, or mild infection resolve on their own and aren’t dangerous. A standard fever in adults is generally defined as a temperature above 100.4°F (38.0°C). Chills that come and go alongside a moderate fever are a normal part of your body’s immune response.
Chills become more concerning when paired with specific symptoms that can indicate sepsis, a dangerous escalation of the body’s response to infection. These include confusion or a major change in mental alertness, rapid shallow breathing, extreme sleepiness or difficulty staying awake, sweating without exertion, and feeling lightheaded. Sepsis can progress to septic shock, which involves a severe drop in blood pressure and organ failure. Confusion or fast breathing alongside chills and fever needs emergency care.
In young children, fever thresholds for concern are lower. Any temperature of 100.4°F (38.0°C) or higher in a baby under three months old is considered high risk for serious illness, regardless of whether chills are present.