A fever is technically defined as a body temperature at or above 100.4°F (38°C). This is the threshold used by the CDC, the American Academy of Pediatrics, and most healthcare providers. But the full picture is more nuanced than a single number, because where you measure, when you measure, and who you’re measuring all shift what counts.
The Standard Threshold
The widely accepted cutoff is 100.4°F (38°C), measured orally. This is the number used in clinical guidelines, hospital screening protocols, and public health definitions. Below that, many providers consider temperatures between 99.5°F (37.5°C) and 100.3°F (37.9°C) to be a low-grade fever, a gray zone that can signal the early stages of infection or inflammation but doesn’t meet the formal definition.
That 100.4°F line isn’t arbitrary. It reflects a meaningful shift in how the body is regulating temperature, one that almost always indicates the immune system has been activated by something, whether an infection, an inflammatory condition, or certain medications.
Why the Number Changes by Measurement Site
Not all thermometers read the same temperature, even when used correctly. The 100.4°F threshold applies to rectal, ear, and temporal artery (forehead) readings. For oral thermometers, a fever starts at 100°F (37.8°C). For armpit readings, the cutoff drops to 99°F (37.2°C), though armpit measurements are the least reliable and are best used as a rough screening tool rather than a definitive reading.
These differences exist because core body temperature (closest to rectal) runs slightly higher than surface or peripheral temperature. If you get an armpit reading that seems borderline, it’s worth rechecking with an oral or ear thermometer.
Your Temperature Fluctuates All Day
Normal body temperature isn’t a fixed 98.6°F. That number, originally established in the 1800s, is more of an average than a rule. Your temperature naturally rises and falls on a daily cycle, hitting its lowest point around 6 a.m. and peaking around 8 p.m. In healthy people, that swing ranges from 0.5°F to 1.9°F over the course of a day. So a reading of 99.1°F at 9 p.m. might be perfectly normal, while the same reading at 6 a.m. could be more meaningful.
This daily rhythm is why a single temperature reading without context can be misleading. A temperature that looks fine in the evening might actually represent an elevated baseline if taken first thing in the morning.
How Fever Works Inside Your Body
A fever isn’t your body malfunctioning. It’s a deliberate response. When your immune system detects an invader like a virus or bacteria, immune cells release signaling molecules that travel to the brain’s temperature control center, a region in the hypothalamus. These signals trigger the production of a chemical messenger that essentially raises the thermostat’s set point.
Once the set point rises, your body acts as though its current temperature is too cold. Blood vessels near the skin constrict to retain heat, sweating decreases, your metabolic rate increases, and you may start shivering. All of this works to push your temperature up to the new, higher target. That’s why you can feel freezing cold even though your body is objectively warmer than usual.
This elevated temperature helps your immune system work more efficiently. Many pathogens reproduce more slowly at higher temperatures, and certain immune cells become more active. Fever is, in most cases, a feature rather than a bug.
Fever Thresholds for Infants
For babies, the same 100.4°F threshold applies, but the urgency is different. The American Academy of Pediatrics flags any temperature at or above 100.4°F in infants between 8 and 60 days old as requiring prompt medical evaluation, even if the baby appears well. Young infants have immature immune systems and can deteriorate quickly, so a fever that would be routine in an adult or older child warrants a call to your pediatrician or a visit to the emergency room.
Why Older Adults Run Cooler
Adults over 65 tend to have lower baseline body temperatures, averaging around 97.7°F in nursing home residents rather than the traditional 98.6°F. This means an older adult can have a significant infection without ever reaching 100.4°F. For this population, a more useful definition of fever is a temperature 1.4°F above that individual’s known normal baseline. When the person’s typical temperature isn’t known, providers often use 99.0°F as a practical threshold.
This matters because delayed recognition of fever in older adults is common. A temperature of 99.2°F might look unremarkable on paper but could represent a meaningful spike for someone who normally sits at 97.5°F.
When a Fever Becomes Dangerous
Most fevers in the 100.4°F to 103°F range, while uncomfortable, aren’t dangerous on their own. The body can tolerate moderate fevers well, and they typically resolve as the underlying illness improves. Fevers above 103°F deserve closer attention, particularly if they persist or are accompanied by confusion, difficulty breathing, or severe pain.
The true danger zone is hyperpyrexia, defined as a temperature above 106.7°F (41.5°C). At this level, the heat itself starts damaging organs. The brain, heart, lungs, liver, and kidneys can all be affected, and complications include brain swelling, permanent brain damage, and coma. Hyperpyrexia is a medical emergency and is rare in typical infections. It’s more commonly associated with heatstroke, certain drug reactions, or conditions affecting the brain’s temperature regulation directly.