Fever chills are the shivering, shaking, and cold sensations your body produces when it’s raising its internal temperature to fight off an infection or illness. Even though your body is actually heating up, you feel cold because your brain has temporarily reset its target temperature higher than normal. The gap between your current body temperature and this new, higher target is what makes you feel freezing and start to shiver.
Why Your Body Shivers During a Fever
Your brain has a built-in thermostat located in a region called the hypothalamus. Under normal conditions, it keeps your core temperature around 98.6°F (37°C). When your immune system detects an invader like a virus or bacteria, it releases chemical signals that push this thermostat’s set point upward, sometimes to 101°F, 103°F, or higher.
Once the set point rises, your brain treats your current normal temperature as too cold. It responds in two ways: it narrows the blood vessels near your skin to trap heat inside your body (which is why your hands and feet may feel icy), and it triggers rapid, involuntary muscle contractions, or shivering, to generate heat. This combination of vasoconstriction and shivering is what produces that distinctive experience of huddling under blankets, teeth chattering, while your thermometer reads a number that says you’re actually warm.
Once your body temperature catches up to the new set point, the chills typically stop. You may then feel hot or flushed. When the fever breaks and the set point drops back to normal, the process reverses: blood vessels open up, you start sweating, and you feel overheated as your body sheds the extra warmth.
What Causes Fever Chills
Infections are the most common trigger. Respiratory infections like the flu, COVID-19, pneumonia, and bronchitis top the list. Urinary tract infections, gastrointestinal bugs, and skin infections can all produce chills as well. In a prospective study of patients presenting with shaking chills, respiratory infections accounted for about 15% of cases, while influenza, urinary tract infections, and gastrointestinal infections each made up 3% to 4%.
Chills accompanied by intense, uncontrollable shaking (sometimes called “rigors”) have traditionally been associated with bacteria in the bloodstream, a condition called bacteremia. While rigors don’t always mean bacteria are present, they tend to signal a more significant immune response than mild chills alone.
Not all chills come from infections. Other triggers include:
- Low blood sugar: your body may shiver as it struggles to maintain energy production
- Drug or alcohol withdrawal: the nervous system becomes hyperactive, producing tremors and chills
- Medication reactions: some drug combinations, particularly those that increase serotonin levels (like certain antidepressants taken alongside migraine medications), can cause shivering, agitation, and temperature changes
- Autoimmune or inflammatory conditions: the same chemical signals that infections trigger can be produced by your own immune system attacking healthy tissue
- Extreme cold exposure: shivering without a fever is simply your body’s normal heating mechanism
Fever Ranges and What They Mean
A fever is generally defined as a temperature at or above 100.4°F (38°C). Harvard Health Publishing breaks adult fevers into three categories:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate: 100.6 to 102.2°F (38.1 to 39.0°C)
- High-grade: 102.4 to 105.8°F (39.1 to 41.0°C)
Chills can occur at any fever level but are most noticeable during the rising phase, when the gap between your actual temperature and your brain’s new set point is greatest. A low-grade fever may produce mild chills or none at all, while a high-grade fever often brings intense shaking.
Managing Fever Chills at Home
Because chills are a symptom of the fever itself, the most effective way to ease them is to bring the fever down. For adults with fevers above 102°F, acetaminophen (Tylenol), ibuprofen (Advil, Motrin), or aspirin can help. Read the label carefully for the correct dose, and avoid doubling up on products that contain the same active ingredient. Many cough and cold medicines already include acetaminophen, so check before adding a separate dose.
For children ages 2 and older with fevers above 102°F, acetaminophen or ibuprofen are appropriate options based on the child’s weight. Aspirin should never be given to children or teenagers due to the risk of a rare but serious condition called Reye’s syndrome. There’s no need to wake a sleeping child to give fever medicine.
Beyond medication, a few practical steps help:
- Stay hydrated. Fever increases fluid loss through sweat and faster breathing. Water, broth, and electrolyte drinks help replace what you’re losing.
- Use light layers. It’s tempting to pile on heavy blankets when you’re shivering, but too much insulation can trap heat and push your temperature higher. A light blanket is enough to take the edge off the chills.
- Rest. Your immune system works more efficiently when your body isn’t spending energy on other activities.
Fever Chills in Infants
Babies under 2 months old are handled very differently from older children. The American Academy of Pediatrics flags any fever at or above 100.4°F (38°C) in infants 8 to 60 days old as something that needs medical evaluation, even if the baby appears well. Their immune systems are immature enough that infections can escalate quickly without obvious warning signs. For infants this young, a rectal thermometer gives the most accurate reading.
Warning Signs That Need Attention
Most fever chills are uncomfortable but not dangerous. They resolve on their own or with basic fever management as the underlying illness runs its course. However, certain patterns warrant prompt medical evaluation.
For adults, a fever that stays at or above 103°F despite taking fever-reducing medication, or any fever lasting longer than three days, should be evaluated by a healthcare professional. The same applies to fevers that keep returning without a clear explanation.
Chills paired with any of the following are more urgent: confusion or difficulty staying alert, trouble breathing, severe headache with a stiff neck, a rash that doesn’t fade when you press on it, pain during urination alongside back or flank pain, or signs of dehydration like very dark urine and dizziness when standing. Intense, uncontrollable shaking (rigors) in someone who looks or feels very unwell can sometimes point to bacteria in the bloodstream, which requires prompt treatment.
For children, a fever that doesn’t respond to medication or lasts longer than three days also warrants a call to their pediatrician. Any fever in an infant younger than 2 months needs same-day evaluation regardless of how the baby seems.