Chills don’t always mean you have a fever, but they often signal that one is on the way. When your body is fighting an infection, chills typically appear during the rising phase of a fever, sometimes minutes to hours before a thermometer would show an elevated reading. A fever in adults is generally defined as a temperature above 100.4°F (38.0°C). But chills can also happen for reasons that have nothing to do with infection or fever at all.
How Chills and Fever Are Connected
When bacteria or viruses enter your body, they trigger the release of chemical signals that essentially trick your brain’s internal thermostat into raising its target temperature. Your brain, specifically a region called the hypothalamus, decides that your normal 98.6°F is too low and resets the goal to something higher, say 101°F or 102°F. At that point, your actual body temperature is below the new target, so your brain treats the situation the same way it would if you walked outside in winter: it makes you shiver.
Shivering is rapid, involuntary muscle contraction that generates heat. Your blood vessels also constrict near the skin surface to reduce heat loss, which is why your hands and feet feel icy cold. You pile on blankets, curl up, and feel freezing, all while your temperature is actively climbing. Once your core temperature reaches the new set point, the chills stop. This is why chills tend to come first and the fever follows.
In certain infections, this pattern is dramatic and unmistakable. Malaria causes a classic cycle where shivering and rigors lasting one to two hours give way to a high fever up to 106°F (41°C). Bacterial pneumonia often begins with sudden shaking chills followed by temperatures of 103°F to 105°F. Even common illnesses like the flu follow this general sequence: chills appear early, then the fever peaks.
Chills Without a Fever
Plenty of situations cause chills with no infection involved. If you take your temperature and it’s normal, one of these may be the reason.
Cold exposure is the most obvious. When your environment drops below a comfortable range, your body shivers to generate warmth. This is the same shivering mechanism as fever-related chills, just triggered by external cold rather than an internal reset.
Intense exercise can also cause chills afterward. During a hard workout, blood flow increases to your skin to help you cool off through sweating. That increased blood flow to the surface persists for some time after you stop exercising, which means your body keeps losing heat even though you’re no longer generating as much. The result is post-workout chills, especially in cool environments or if your clothing is damp with sweat.
Low blood sugar (hypoglycemia) triggers chills in some people, particularly those with diabetes. Emotional stress and anxiety can do it too. Psychological stress activates a separate pathway in the hypothalamus that raises heart rate, constricts blood vessels, and can produce shivering through skeletal muscle activation. This is a distinct process from infection-driven fever, though it can sometimes raise body temperature slightly on its own. Panic attacks, PTSD, and chronic stress conditions are all associated with this kind of response.
Other non-fever causes include hormonal shifts during menopause (hot flashes often alternate with chills), hangovers, and drug or alcohol withdrawal.
Medications That Cause Chills
Some medications can trigger both chills and fever as a side effect, a phenomenon known as drug fever. This isn’t an infection. It’s your immune system reacting to the medication itself. Common culprits include antibiotics (especially penicillin-type drugs and cephalosporins), anti-seizure medications, certain blood pressure drugs, and even over-the-counter painkillers like ibuprofen. Between 36% and 51% of drug fever cases include shaking chills, which means their presence alone can’t distinguish a drug reaction from a true infection. If you recently started a new medication and develop unexplained chills and fever, that timing is worth noting.
Mild Chills vs. Shaking Chills
Not all chills carry the same weight. Clinically, there’s a meaningful difference between feeling cold and shivery versus full-body shaking that you can’t control even under heavy blankets. The latter, called rigors, is a much stronger signal of serious infection.
A large analysis of studies found that shaking chills made bacteremia (bacteria in the bloodstream) about 12 times more likely compared to people without chills. Even moderate chills raised the risk roughly fourfold. Mild chills, the kind where you just feel cold and a little shivery, showed a much weaker association. This grading matters because bloodstream infections can progress to septic shock if antibiotics are delayed more than five hours, and delays beyond 12 hours are linked to higher mortality.
So if you or someone you’re caring for has violent, uncontrollable shaking with a high fever, that combination deserves urgent attention. This is especially true if it’s accompanied by confusion, rapid breathing, a stiff neck, or a rash that doesn’t fade when you press on it.
What to Do When You Get Chills
The first practical step is simple: take your temperature. If it’s above 100.4°F (38.0°C), you have a fever. If it’s normal now but you feel chills, check again in 30 to 60 minutes, since chills often precede the temperature spike.
For children, the thresholds shift with age. In babies under three months, any temperature above 99.4°F (37.4°C) is considered concerning. For children between 3 and 36 months, temperatures above 101.3°F (38.5°C) fall into the high-fever category. These lower thresholds exist because young infants have immature immune systems and a fever can signal something serious before other symptoms appear.
If your temperature is normal and stays normal, think about context. Did you just finish exercising? Are you anxious or stressed? Have you been in a cold environment? Are you on a new medication? Skipped a meal? These everyday explanations account for most fever-free chills. Persistent or recurring chills without an obvious cause, though, are worth investigating, particularly if they come with fatigue, weight loss, or night sweats, which can point to conditions that need a proper workup.