A fever happens when your brain’s internal thermostat, located in a region called the hypothalamus, resets itself to a higher temperature. Instead of maintaining the usual 98.6°F (37°C), your body defends a new, elevated target, typically above 100.4°F (38°C). This reset is almost always triggered by your immune system responding to an infection, though injuries, medications, and inflammatory conditions can cause it too.
How Your Brain Resets Its Thermostat
The hypothalamus constantly monitors your body temperature using signals from temperature sensors throughout your body and in the brain itself. Under normal conditions, it keeps your temperature stable by balancing heat production with heat loss through sweating, blood flow to the skin, and metabolic activity.
When your immune system detects a threat, like bacteria or a virus, white blood cells release signaling molecules called cytokines. The key players include IL-1, IL-6, and TNF-alpha, all of which act directly on the hypothalamus. These molecules essentially tell your thermostat that 98.6°F is no longer the goal. The new set point might be 101°F, 103°F, or higher, depending on the severity of the immune response.
Once the set point rises, your brain treats your current normal temperature as “too cold” and activates the same warming mechanisms it would use if you stepped outside in winter. That’s why the early stage of a fever feels so counterintuitive: you’re getting hotter, but you feel freezing.
Why You Shiver and Feel Cold
To close the gap between your actual temperature and the new, higher set point, your body does two things simultaneously: it generates more heat and prevents heat from escaping. Blood vessels near your skin constrict, pulling warm blood away from the surface. That’s why your hands and feet may feel icy even though your core temperature is climbing. Your metabolism ramps up to burn more energy as heat.
Shivering is the most aggressive heat-generating tool your body has. Rapid, involuntary muscle contractions produce a burst of warmth, which is why the onset of a high fever often comes with what doctors call “shaking chills.” You may also instinctively pile on blankets or curl up, a behavioral response to conserve heat that your hypothalamus drives just as it drives shivering.
Once your body temperature reaches the new set point, the chills typically stop. You feel hot instead. When the immune response winds down and the set point drops back to normal, the process reverses: blood vessels dilate, you start sweating, and you may kick off the covers. That “fever breaking” sensation is your body dumping the excess heat it no longer needs.
What Triggers the Immune Response
Infections are the most common cause. Bacteria, viruses, fungi, and parasites all contain molecules that your immune system recognizes as foreign. Bacterial infections tend to produce especially robust fevers. Compounds on the surface of bacteria, called endotoxins, trigger a characteristic pattern where the fever peaks within about 90 minutes or rises in two waves.
But infections aren’t the only trigger. Roughly 10% to 30% of unexplained fevers turn out to be caused by non-infectious inflammatory conditions, including autoimmune diseases like lupus, rheumatoid arthritis, and Crohn’s disease. In these cases, your immune system is reacting to your own tissues rather than an outside invader, but the fever pathway is identical: cytokines signal the hypothalamus, and the set point goes up.
Other non-infectious causes include certain medications (sometimes called drug fever), blood clots, thyroid inflammation, and rarely, cancers like lymphoma. About 10% to 20% of persistent unexplained fevers fall into this miscellaneous category.
Why Your Body Bothers With Fever
Fever isn’t a malfunction. It’s a deliberate immune strategy. Research from the National Institutes of Health has shown that T cells, a critical part of your immune defense, work measurably better at fever temperatures. When researchers cultured mouse T cells at 102.2°F (39°C) instead of the normal 98.6°F (37°C), all types of T cells multiplied faster. T helper cells, which coordinate the broader immune response, also released more signaling molecules at the higher temperature.
Many pathogens also reproduce more slowly in a warmer environment. So fever simultaneously boosts your immune system’s offensive power while weakening the invader. This is part of why moderate fevers in otherwise healthy people are generally considered a sign that the immune system is doing its job rather than something that needs to be suppressed immediately.
Temperature Thresholds That Matter
A fever is formally defined as a body temperature above 100.4°F (38°C). Most fevers from common infections stay in the 100°F to 103°F range and resolve within a few days.
The danger zone starts at 106.7°F (41.5°C), a condition called hyperpyrexia. At this level, the heat itself can damage organs and is considered a medical emergency. Fevers from infections alone rarely reach this point. Hyperpyrexia more commonly results from drug reactions, heatstroke, or central nervous system problems where the hypothalamus itself is compromised.
For infants, the stakes are different. Any fever above 100.4°F in a baby under 3 months old warrants prompt medical evaluation, because young infants have immature immune systems and fewer visible signs when something serious is happening. Babies under 21 days old with a fever typically receive a full workup and monitoring in a hospital setting as a precaution.
How to Measure Accurately
Rectal thermometers are the most accurate, which is why they remain the standard for infants. For adults and older children, oral thermometers provide similar accuracy and are far more practical. Ear thermometers can be thrown off by earwax, ear infections, or improper positioning. Forehead (temporal) thermometers are convenient but tend to be less reliable, especially outdoors, in cold air, or when the forehead is sweaty.
Temperatures vary slightly depending on where you measure, and there’s no reliable formula for converting between sites. If you’re tracking a fever over time, the most useful approach is to use the same method and the same thermometer each time so you can spot meaningful changes rather than measurement noise.