A fever is your body deliberately raising its own temperature above the normal range, typically to fight off an infection. The widely accepted threshold is 100.4°F (38°C), measured orally or rectally. While fever often feels alarming, it’s not an illness itself. It’s a coordinated defense response, one that helps your immune system work more effectively against whatever triggered it.
How Your Body Creates a Fever
Your brain contains a built-in thermostat, a cluster of neurons that constantly monitors and adjusts your core temperature. Under normal conditions, this thermostat keeps you around 97–99°F by balancing heat production with heat loss. When your immune system detects a threat, like bacteria or a virus, it releases chemical signals that essentially reprogram the thermostat to a higher setting.
Once the set point rises, your body treats its current normal temperature as “too cold.” That’s why you shiver and feel chilled at the start of a fever: your muscles contract rapidly to generate heat, blood vessels near the skin tighten to reduce heat loss, and you instinctively reach for blankets. Your body is working hard to reach the new, higher target. When the infection starts clearing and those chemical signals fade, the thermostat resets back down, and you suddenly feel hot and sweaty as your body dumps the excess heat.
Why Fever Actually Helps
Fever isn’t a malfunction. It’s a strategy that evolved across nearly all vertebrates, which suggests it carries a real survival advantage. The protective benefit works through several mechanisms at once.
The most straightforward one: raising your body temperature pushes many pathogens out of their comfort zone. Bacteria and viruses that thrive at your normal body temperature replicate less efficiently when things heat up. Human coronavirus 229E, for instance, shows reduced replication when temperatures climb from the low 90s (Fahrenheit) to normal body temperature and beyond. Rhinovirus, the most common cause of colds, also struggles as temperature rises.
But fever does more than just make life harder for germs. It actively boosts your immune system’s performance. At fever-range temperatures, your body increases production of interferons, the proteins that serve as an early warning system against viruses. Immune cells travel to infection sites more efficiently because heat-sensitive proteins on their surface shift into a more active shape, improving their ability to reach where they’re needed. Fever also enhances the ability of specialized cells to present pieces of invading organisms to the rest of the immune system, essentially improving communication between the troops. Antibody production increases, and certain types of T cells differentiate more effectively.
Some of these benefits have specific temperature thresholds. The virus-sensing machinery inside your cells works noticeably better at 98.6°F than at the slightly cooler temperatures found in your nasal passages (around 91°F), and performance improves further at fever temperatures. That said, the benefits have limits. At very high temperatures (above 107°F), some of these same immune sensors start to lose function.
Fever Severity Levels
Not all fevers are equal. Harvard Health Publishing breaks them into three ranges:
- Low-grade: 99.1 to 100.4°F (37.3 to 38.0°C)
- Moderate-grade: 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)
Low-grade fevers are extremely common and often accompany minor viral infections. Most healthy adults handle moderate fevers without complications. High-grade fevers deserve closer attention, particularly if they persist for more than a day or two, but they aren’t automatically dangerous in an otherwise healthy person.
Above 106.7°F (41.5°C) is a category called hyperpyrexia, and it is a medical emergency. At this level, the heat itself starts damaging organs. Your brain, heart, lungs, liver, and kidneys can all struggle to function. Symptoms include confusion, seizures, a racing heart, muscle rigidity, and loss of consciousness. Hyperpyrexia can cause permanent brain damage or death without rapid treatment. Fortunately, infections alone rarely push temperatures this high. Hyperpyrexia more commonly results from heatstroke, certain drug reactions, or anesthesia complications.
Common Causes Beyond Infections
Most fevers are triggered by infections: colds, flu, urinary tract infections, ear infections, and the like. But infection isn’t the only cause. In cases of prolonged unexplained fever, studies show that noninfectious inflammatory diseases account for 10 to 30 percent of cases. These include autoimmune conditions like lupus, rheumatoid arthritis, and a condition called adult Still disease, as well as inflammatory bowel conditions like Crohn disease and vascular inflammation like giant cell arteritis.
Another 10 to 20 percent of unexplained fevers fall into a miscellaneous category that includes medication side effects, thyroid inflammation, and blood clots. Some medications can trigger fever as a reaction, and in rare cases, people present with what’s called factitious fever, meaning the fever is self-induced. If you’ve had a fever lasting more than three weeks without a clear cause, that pattern has a specific clinical name (fever of unknown origin) and typically warrants a thorough workup.
How to Measure Temperature Accurately
Where you place the thermometer matters. Rectal readings are considered the gold standard, especially for infants, because they most closely reflect core body temperature. Oral readings are reliable for older children and adults. Ear (tympanic) thermometers are convenient but slightly less precise. Research from Johns Hopkins found that ear thermometers correlated well with oral and rectal readings in most children, with about 80% sensitivity for detecting fever, but they were notably less reliable in infants under three months old.
Armpit (axillary) readings run lower than core temperature. The Mayo Clinic considers an armpit reading of 99°F (37.2°C) or higher to be a fever, compared to 100.4°F for oral or rectal measurements. If you’re using an armpit thermometer and get a borderline reading, an oral or rectal check will give you a more definitive answer.
Fever in Infants and Children
Age changes the equation significantly. A fever that’s routine in a five-year-old can be serious in a newborn. The American Academy of Pediatrics recommends calling your pediatrician immediately if a baby under two months old has a rectal temperature of 100.4°F or higher. Young infants have immature immune systems, and fever at this age can signal a serious bacterial infection that needs rapid evaluation.
For babies under three months, additional warning signs that warrant an immediate call include inconsolable crying, difficulty waking, neck stiffness, purple spots on the skin, breathing difficulty that doesn’t improve after clearing the nose, inability to swallow, or drooling. In any child, a temperature reaching 105°F is considered a medical emergency requiring immediate attention.
Older children and adults generally tolerate fevers well. The height of the fever alone doesn’t always predict how serious the underlying illness is. A child with a 103°F temperature who’s drinking fluids and playing between bouts of feeling crummy is often in better shape than one with a lower fever who seems listless and unresponsive. How the person looks and acts matters as much as the number on the thermometer.