A fever happens when your body deliberately raises its own temperature above the normal range, usually in response to an infection. The widely used clinical threshold is 100.4°F (38°C), as defined by the CDC, though some sources consider an oral reading of 100°F or above worth noting. A fever isn’t something that attacks you from the outside. It’s your body’s own defense system kicking into a higher gear.
What Happens Inside Your Body
Your brain has a built-in thermostat located in a small region called the hypothalamus. Normally, it keeps your core temperature hovering around 98.6°F. When your immune system detects a threat, it releases signaling chemicals (a type of protein called cytokines) that travel through your bloodstream and essentially tell the hypothalamus to crank the dial up. The hypothalamus responds by resetting its target temperature to a higher level.
Once that new set point is established, your body treats its current normal temperature as “too cold” and starts working to warm up. Blood vessels near the skin constrict to hold heat in, which is why you might look pale or feel chilled even though your temperature is climbing. Your metabolism speeds up to generate more heat. If the gap between your current temperature and the new set point is large enough, you’ll start shivering, sometimes intensely enough to cause what people call “the chills.” All of this is your body doing exactly what the hypothalamus is telling it to do.
Why Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate strategy. Raising your body temperature makes your internal environment less hospitable to many bacteria and viruses by pushing them outside their ideal temperature range, which slows their ability to reproduce. At the same time, the higher temperature boosts your immune system in several ways: white blood cells move through your body more efficiently, your cells get better at presenting pieces of invaders to the immune system so it can target them, and your body produces antibodies more effectively. Even the way immune cells physically stick to blood vessel walls and migrate to infection sites improves at fever temperatures.
This is why mild to moderate fevers are generally considered a sign that your immune system is doing its job, not a sign that something has gone terribly wrong.
The Most Common Causes
Infections are by far the most frequent trigger. Virtually any type of germ can cause a fever.
- Viral infections are the most common culprit in everyday life. Colds, the flu, COVID-19, stomach bugs, and infections like mononucleosis (caused by Epstein-Barr virus) all routinely cause fevers.
- Bacterial infections include strep throat, urinary tract infections, sinus infections, ear infections, pneumonia, and dental abscesses. These fevers can sometimes run higher and last longer than viral fevers.
- Other infections from fungi or parasites can also trigger fevers, though these are less common in otherwise healthy people.
Infections aren’t the only cause, though. Your immune system can also trigger a fever when there’s no germ involved at all. Autoimmune conditions like lupus, rheumatoid arthritis, and Crohn’s disease can cause recurring or persistent fevers because the immune system is chronically activated against the body’s own tissues. Certain inflammatory conditions like sarcoidosis and giant cell arteritis do the same thing.
Other non-infectious triggers include reactions to medications, some cancers (particularly blood cancers like lymphoma), and severe sunburn or heat exhaustion. Vaccines commonly cause low-grade fevers as well, because they’re designed to activate the same immune response that infections do, just in a controlled way.
How to Measure a Fever Accurately
Not all thermometers give you the same reading, and where you take your temperature matters. Rectal thermometers are considered the most accurate, which is why they’re recommended for infants and young children. Oral thermometers provide similar accuracy for older children and adults and are far more practical for everyday use.
Forehead (temporal) thermometers are convenient but less reliable. Direct sunlight, cold air, and even a sweaty forehead can throw off the reading. Ear thermometers can also be inaccurate if there’s earwax buildup, an ear infection, or if the probe isn’t positioned correctly in the ear canal.
One important point: there’s no reliable formula for converting a temperature taken at one body site to match another. An armpit reading of 99°F doesn’t translate neatly to a specific oral or rectal number. The best approach is to use the same method each time so you can track whether your temperature is rising or falling consistently.
What a Fever Feels Like
The experience of a fever follows a predictable pattern that maps directly to what’s happening with your hypothalamus. In the rising phase, when your set point has been raised but your body hasn’t caught up yet, you feel cold. You might shiver, pile on blankets, and curl up. Your hands and feet may feel icy because blood is being diverted away from your skin to conserve heat.
Once your body reaches the new set point, the chills usually stop and you feel hot, flushed, and fatigued. Your heart rate increases. You may feel achy, lose your appetite, and become more sensitive to light. These symptoms aren’t caused by the fever itself so much as by the same immune chemicals that triggered the fever in the first place.
When the fever breaks, either because your immune system is winning or because you took a fever-reducing medication, the set point drops back down. Now your body needs to shed heat quickly, so blood rushes to the skin and you start sweating, sometimes heavily. This is often the point where people say they “feel better” even before the illness is fully resolved.
Fevers in Babies and Young Children
The same 100.4°F (38°C) threshold applies to infants, but the stakes are different. The American Academy of Pediatrics flags any fever at or above 100.4°F in babies between 8 and 60 days old as something that needs medical evaluation, even if the baby looks well. Young infants have immature immune systems, so a fever can be the only visible sign of a serious bacterial infection. In babies under 8 weeks, a fever is treated as urgent regardless of how the baby appears.
Older children handle fevers much the way adults do. The number on the thermometer matters less than how the child is acting. A child with a 102°F fever who is drinking fluids and playing is generally in better shape than a child with a 100.5°F fever who is limp, unresponsive, or refusing to drink.
Why Some People Run Hotter Than Others
Normal body temperature isn’t a fixed number. It varies by person, time of day, age, and activity level. Your temperature is typically lowest in the early morning and highest in the late afternoon, with a swing of about one degree throughout the day. Women’s temperatures also fluctuate with their menstrual cycle, running slightly higher after ovulation. Older adults tend to have lower baseline temperatures, which means a reading of 99°F in an elderly person may represent a more significant immune response than it would in a younger adult.
This natural variation is why context matters more than a single number. Tracking your own baseline over time gives you a much better sense of when something is off than relying on the textbook 98.6°F, a figure that was established in the 1800s and has since been shown to run a bit high for most modern populations.