A fever happens when your brain deliberately raises your body’s internal temperature, almost always in response to an infection. It’s not a malfunction. Your body detects a threat, sends chemical signals to the brain, and the brain resets your internal thermostat to a higher target, typically above 100.4°F (38°C). Everything that follows, the chills, the shivering, the crawling under blankets, is your body working to reach that new, higher temperature on purpose.
How Your Brain Controls Temperature
A small region at the base of your brain called the hypothalamus acts as your body’s thermostat. It constantly receives signals from temperature sensors in your skin and internal organs, then adjusts your body accordingly. When you’re healthy, it keeps your core temperature hovering around 98.6°F (37°C), give or take about a degree depending on the time of day, your activity level, and your individual baseline.
The hypothalamus maintains this balance through two opposing sets of neurons. One group responds to heat by suppressing heat production, effectively cooling you down. The other group responds to cold by ramping up heat production. Under normal conditions, these two systems keep each other in check. When a fever begins, the balance shifts: the brain’s thermostat gets chemically pushed to a higher set point, and your body behaves as though it’s too cold, even if you’re at a perfectly normal room temperature.
The Chain Reaction That Starts a Fever
The process begins when your immune system detects something foreign, most commonly a virus or bacterium. Components of these invaders, like the lipopolysaccharide found on the outer membrane of many common bacteria, act as the initial trigger. Your immune cells recognize these foreign substances and respond by releasing signaling molecules called cytokines, particularly interleukin-1, interleukin-6, and tumor necrosis factor. These cytokines are the real drivers of fever.
Once these cytokines reach the brain, they trigger the production of a small molecule called prostaglandin E2 near the hypothalamus. This is the chemical that actually raises the thermostat’s set point. With the set point now at, say, 102°F instead of 98.6°F, your brain perceives your current normal temperature as too low. It responds the same way it would if you walked outside in winter: blood vessels near the skin constrict to trap heat, your muscles begin to shiver to generate warmth, and you feel an intense urge to bundle up. This is why fevers typically start with chills, even though your temperature is already climbing.
This is also why fever-reducing medications work the way they do. They block the production of prostaglandin E2, which lets the hypothalamus reset back to its normal set point. Your body then switches into cooling mode: you start sweating and flushing as blood rushes to the skin to release heat.
Infections Are the Most Common Cause
The vast majority of fevers come from infections. Viral infections like colds, flu, and COVID-19 tend to produce lower-grade fevers alongside widespread symptoms like body aches, fatigue, and congestion. Bacterial infections, including strep throat, urinary tract infections, and pneumonia, can produce higher or more persistent fevers. One pattern worth knowing: a viral illness that seems to improve and then suddenly gets worse, with a higher fever and new symptoms like localized pain, can signal that a secondary bacterial infection has taken hold.
Gram-negative bacteria are particularly potent fever triggers because of the lipopolysaccharide (endotoxin) embedded in their cell walls. Gram-positive bacteria like Staphylococcus aureus and Streptococcus produce their own toxins that can stimulate an especially aggressive immune response, sometimes called a superantigen reaction, which can cause very high fevers.
Fevers Without Infection
Not all fevers come from germs. Your immune system can also trigger the same cytokine cascade in response to non-infectious threats. Autoimmune conditions like lupus, rheumatoid arthritis, and sarcoidosis can cause recurring or persistent fevers because the immune system is chronically activated against the body’s own tissues. Certain cancers, particularly lymphomas and leukemias, are known to produce fevers through the same cytokine pathways.
Some medications can also cause fevers as a side effect. Drug fever has been documented with a wide range of medications, including some blood pressure drugs, blood thinners, seizure medications, diuretics, and even antihistamines. The fever usually resolves once the medication is stopped, though it can take a few days. There are also substances produced inside the body, like antigen-antibody complexes and certain inflammatory compounds, that can trigger a fever without any external infection or drug involved.
Fever Is Not the Same as Overheating
There’s an important distinction between a true fever and hyperthermia, which is the medical term for dangerous overheating. A fever is a controlled, intentional rise in temperature orchestrated by the brain. Your thermostat has been turned up, and your body is carefully regulating itself to reach that higher target. Hyperthermia is the opposite: your body’s cooling systems have been overwhelmed, and temperature rises in an uncontrolled way. Heat stroke, thyroid emergencies, and certain drug reactions can all cause hyperthermia.
This distinction matters practically. Fever-reducing medications work on fevers because they target the prostaglandin pathway in the brain. They don’t work on hyperthermia, because the hypothalamus isn’t the problem. Someone with heat stroke needs external cooling, not medication.
Why Your Body Does This on Purpose
Fever evolved as a defense mechanism, and it’s remarkably old. Fish, reptiles, and amphibians all exhibit fever-like responses to infection, raising their body temperature by seeking warmer environments. In mammals, the elevated temperature serves at least two functions: many pathogens reproduce more slowly at higher temperatures, and immune cells become more active and mobile in a warmer environment. The fever essentially creates conditions that favor your immune system and handicap the invader.
This is part of why moderate fevers in otherwise healthy people are generally not harmful in themselves. The discomfort is real, but the process is doing something useful. Very high fevers above 104°F (40°C), however, can become dangerous in their own right, particularly in young children, older adults, and people with chronic health conditions.
Where and How You Measure Matters
Not all thermometers give the same reading. Rectal temperatures are considered the most accurate noninvasive measurement, especially in children under three. Temporal artery (forehead) readings tend to run about 0.2°C lower than rectal, while armpit readings run nearly a full degree Celsius lower. In one study comparing methods in children, armpit thermometers detected only about 12% of fevers that rectal thermometers caught, while temporal artery thermometers caught about 62%.
For adults, oral thermometers are the most practical option and are reasonably accurate. The standard threshold for fever is 100.4°F (38°C) measured orally, rectally, or at the temporal artery. For armpit readings, 99°F (37.2°C) is considered the fever threshold because of the consistently lower readings at that site.
Fevers That Need Immediate Attention
Most fevers in healthy adults and older children resolve on their own within a few days. But certain combinations of fever and other symptoms signal something potentially serious. A fever with a stiff neck, severe headache, and confusion can indicate meningitis, an infection of the membranes surrounding the brain. Additional warning signs include sensitivity to light, a dark purple rash, rapid breathing, and severe pain in the muscles, joints, or abdomen.
In babies, the rules are stricter. Any fever in an infant younger than three months warrants a call to a doctor, regardless of how the baby seems otherwise. For babies three to six months old, a fever above 100.4°F or any fever accompanied by signs of illness like poor feeding or unusual irritability needs medical evaluation. For children six months to two years, a fever above 100.4°F that lasts more than a day is the threshold. In babies, fever symptoms can look different than in adults: instead of complaining of stiffness or pain, a sick infant may feed poorly, seem unusually sluggish, have abnormal reflexes, or show a bulging soft spot on the skull.