A fever is a temporary rise in body temperature, typically above 100.4°F (38°C), that signals your immune system is actively fighting an infection or illness. It’s not a disease itself but a defense mechanism, one your body deliberately triggers to make conditions hostile for invading bacteria or viruses. Most fevers resolve on their own within a few days and, despite how miserable they feel, are doing exactly what they’re supposed to do.
What Counts as a Fever
The standard medical threshold for a fever is a core body temperature of 100.4°F (38°C) or higher. But the number you see on a thermometer depends on where you take the reading. An oral temperature of 100°F (37.8°C) or higher qualifies as a fever, while an armpit reading of 99°F (37.2°C) or higher does too, since armpit measurements run lower than internal temperatures.
That familiar “normal” of 98.6°F actually comes from data published in 1868. A Stanford Medicine analysis of more than 618,000 temperature readings found that today’s average adult body temperature is closer to 97.9°F, with normal values ranging from 97.3°F to 98.2°F. Your personal baseline shifts depending on your age, sex, weight, and the time of day. Body temperature tends to be lowest in the morning and peaks in the late afternoon, so a reading of 99°F at 7 a.m. is more meaningful than the same number at 5 p.m.
How Your Body Creates a Fever
Fever starts in a small region at the base of your brain that acts as your body’s thermostat. When you get an infection, your immune cells detect the invader and release signaling molecules into the bloodstream. These molecules travel to the brain’s temperature-control center and trigger the production of a chemical messenger called prostaglandin E2. That messenger essentially turns the thermostat dial up, resetting your “normal” to a higher target, say 102°F instead of 98°F.
Once the set point rises, your body treats its current temperature as too cold. This is why you feel chilled and start shivering even though your skin may be warm to the touch. Blood vessels near the skin constrict to trap heat inside, your muscles contract rapidly (shivering) to generate warmth, and you instinctively reach for a blanket. Your temperature keeps climbing until it reaches the new, higher set point. Common fever-reducing medications work by blocking the production of that prostaglandin messenger, which brings the thermostat back down.
Why Fever Helps You Fight Infection
A fever isn’t just a side effect of being sick. It’s a coordinated immune strategy with real benefits. Higher body temperatures boost nearly every branch of your immune response. White blood cells move faster, engulf pathogens more effectively, and produce more of the reactive molecules that destroy bacteria. Natural killer cells, which target virus-infected cells, ramp up their activity. Your body also increases production of interferons, proteins with specific antiviral properties.
At the same time, the heat puts direct pressure on the invaders. Pathogens that are actively dividing are especially vulnerable to temperature stress because even a few degrees above their optimum can damage their proteins, disrupt their cell membranes, and interfere with DNA replication. Your own cells, which aren’t dividing as rapidly at the infection site, tolerate the heat better. Fever essentially tilts the playing field: it supercharges your defenses while weakening the enemy.
What a Fever Feels Like, Stage by Stage
A fever typically moves through three distinct phases, each with its own set of symptoms.
During the rising phase, your thermostat has been reset upward but your body hasn’t caught up yet. You feel cold, may have goosebumps, and often experience chills or shivering. A sudden, intense episode of teeth-chattering, bed-shaking chills (called a rigor) can happen when the temperature shoots up quickly. Muscle aches, headache, and general fatigue often set in during this stage.
At the plateau, your temperature has reached the new set point. The chills usually stop, and you may feel flushed and hot instead. Your skin looks red, you feel warm to the touch, and you’re likely fatigued with a poor appetite. Heart rate and breathing may be slightly faster than usual, since your metabolism is running at a higher pace.
During the breaking phase, the thermostat resets back to normal, often because your immune system is gaining control of the infection. Now your body needs to shed excess heat. Blood vessels near the skin dilate, and you start sweating, sometimes heavily. You may feel suddenly drenched but also relieved, as the uncomfortable heat sensation fades.
Where to Take Your Temperature
Not all thermometer placements are equally reliable. Rectal readings are the closest to true core body temperature, which is why they remain the gold standard for infants and young children. Oral thermometers are convenient for older children and adults, but they can read about 1°F lower than rectal on average, and the gap between the two can be even wider in individual cases (as much as nearly 3°F lower). Ear (tympanic) thermometers are fast but can be thrown off by earwax or a curved ear canal. Armpit readings are the least accurate and are best used as a quick screen rather than a definitive measurement.
If you get an armpit or forehead reading that seems borderline, it’s worth confirming with an oral or rectal measurement before deciding how to respond.
Fever in Babies and Children
The same 100.4°F (38°C) threshold applies to children when measured rectally, by ear, or with a temporal artery (forehead) thermometer. Oral readings count as a fever at 100°F, and armpit readings at 99°F. For infants under 3 months old, any rectal temperature of 100.4°F or higher is treated seriously regardless of how the baby looks or acts, because very young infants can deteriorate quickly and their immune systems are still immature.
Babies with fever may not show the classic signs adults recognize. Instead of complaining of chills, they may seem unusually sleepy or irritable, feed poorly, or have abnormal reflexes. A bulging soft spot on the skull can also be a sign of a serious infection in an infant.
When a Fever Needs Medical Attention
Most fevers in otherwise healthy adults are caused by common viral infections and don’t require medical treatment beyond rest, fluids, and comfort measures. The temperature itself becomes more concerning above 104°F (40°C) in adults, which warrants a call to your doctor.
The bigger concern isn’t usually the number on the thermometer but what comes with it. Certain accompanying symptoms can signal something more serious, like meningitis or a bloodstream infection. These include:
- Stiff neck combined with headache and sensitivity to light
- Confusion or altered mental state
- Seizure
- Trouble breathing
- Severe pain anywhere in the body
- A dark purple rash that doesn’t fade when you press on it
- Rapid breathing with cold hands and feet
These combinations can start out looking like a routine flu-like illness and worsen rapidly. In infants, the warning signs look different: persistent vomiting, extreme lethargy, poor feeding, or a bulging soft spot all warrant immediate evaluation.