A fever is a temporary rise in body temperature, typically above 100°F (37.8°C) when measured orally. It’s not an illness itself but a sign that your body’s immune system is actively fighting something, most often an infection. Your brain’s internal thermostat, normally set around 98.6°F, shifts upward in response to signals from your immune system, generating the heat, chills, and discomfort you feel when you’re sick.
Why Your Body Raises Its Temperature
Fever exists because it works. When your body temperature climbs, your immune cells become significantly more active. Research from the National Institutes of Health found that T cells, a critical part of your immune defense, proliferate faster and produce more signaling molecules at fever temperature (102.2°F) compared to normal body temperature. The T cells that survive the higher temperature develop more energy-producing structures inside them, making them better equipped to fight off invaders.
This enhanced immune activity comes at a cost. Your heart beats faster, your breathing rate increases, and your metabolism burns through more energy. That’s why fevers leave you feeling exhausted and achy. Your muscles may shiver to generate additional heat, which is why you can feel cold even as your temperature climbs. These are all signs your body is working hard, not signs that something has gone wrong with the fever itself.
Common Causes Beyond Infections
Most fevers are caused by viral or bacterial infections: colds, flu, urinary tract infections, ear infections, or stomach bugs. Your immune system detects the invader and releases chemicals that tell your brain to turn up the heat. But infections aren’t the only trigger.
Autoimmune conditions like rheumatoid arthritis and lupus can cause recurring fevers because the immune system is chronically activated, even without an infection to fight. Certain medications can also raise body temperature or impair your body’s ability to cool itself. Psychiatric medications, including antipsychotics and some antidepressants, are common culprits. So are antihistamines, seizure medications, stimulants, and even thyroid replacement hormones. Severe heat exposure, some cancers, and inflammatory conditions round out the list of non-infectious causes.
When a fever persists for weeks without an obvious explanation despite medical evaluation, doctors classify it as a “fever of unknown origin.” This designation simply means the cause requires deeper investigation, not that it’s untreatable.
How to Measure Temperature Accurately
Not all thermometers are equally reliable, and where you measure matters. A study comparing multiple thermometer types against a medical gold standard found that ear (tympanic) thermometers were the most accurate consumer option, with readings that deviated by less than a tenth of a degree on average. Oral (under-the-tongue) thermometers came in close behind, typically reading about 0.2°C lower than actual core temperature.
Forehead thermometers are convenient but less consistent. Depending on the brand, they can read anywhere from 0.2°C low to 0.4°C high compared to core temperature, and their margin of error is wider. Thermal imaging cameras, sometimes used for screening in public spaces, were the least accurate of all devices tested.
For the most reliable reading at home, an ear or oral thermometer is your best bet. If you use a forehead scanner, keep in mind that a single reading may be off by nearly a full degree Fahrenheit in either direction. Taking two or three readings and noting the trend gives you a more useful picture than relying on one number.
Normal Fever vs. Dangerous Fever
Most fevers in adults and older children are uncomfortable but not dangerous. A temperature of 100°F to 103°F with a known cause like a cold or flu is your body doing exactly what it’s designed to do. You can manage the discomfort with rest, fluids, and over-the-counter fever reducers if you choose, though bringing the temperature down doesn’t speed up recovery from the underlying illness.
The situation changes at the extremes. Hyperpyrexia, defined as a body temperature above 106.7°F (41.5°C), is a medical emergency. At that level, the heat itself starts damaging organs and brain tissue. Warning signs include confusion, rapid heart rate, loss of consciousness, seizures, and severe muscle stiffness. Anyone with a temperature in this range needs emergency care immediately.
Fever in Infants and Young Children
The rules are different for babies. Any infant under two months old with a rectal temperature of 100.4°F (38°C) or higher needs immediate medical evaluation, even if the baby otherwise seems fine. Young infants don’t have fully developed immune systems, so a fever can signal a serious infection that progresses quickly. This is one of the clearest, most consistent guidelines in pediatric medicine: don’t wait and see with a fever in a newborn.
Febrile seizures are another concern unique to children. These occur in 2% to 5% of kids between six months and five years old, triggered by the rapid rise in temperature rather than how high the fever gets. A child having a febrile seizure may stiffen, shake, or lose consciousness. Most episodes end on their own within a few minutes and don’t cause lasting harm. If a seizure lasts longer than five minutes, call 911. Children who experience one febrile seizure have a higher chance of having another, but the vast majority outgrow them entirely and suffer no long-term effects.
What a Fever Feels Like, Stage by Stage
A fever typically moves through three phases. In the first, your brain has reset your thermostat upward but your body hasn’t caught up yet. You feel cold, you shiver, and you want to bundle up under blankets. Your actual temperature is climbing during this stage.
In the second phase, your temperature has reached its new set point. The chills stop, and you feel hot, flushed, and fatigued. Your skin may look red, and you’ll likely feel thirsty as your body loses fluid through sweating and increased breathing. This is the plateau, and it can last hours or days depending on the cause.
The third phase is the break. Your thermostat resets back to normal, and your body needs to dump the excess heat. You sweat, sometimes heavily. You may feel suddenly better or simply drained. Staying hydrated through all three phases matters more than any medication you take, because the combination of sweating, faster breathing, and increased metabolism can quietly push you toward dehydration.