A “sick temperature,” or fever, starts at 100.4°F (38°C). That’s the threshold most doctors use for both adults and children. Anything above that point generally signals your body is fighting an infection or illness. Below that but above your normal baseline, you may be running what’s called a low-grade fever.
Fever Ranges in Adults
Normal body temperature averages 98.6°F (37°C) when measured by mouth, though it naturally fluctuates throughout the day. Your temperature tends to be lowest in the early morning and highest in the late afternoon. A reading slightly above 98.6°F doesn’t necessarily mean you’re sick.
Harvard Health Publishing breaks fever into three tiers:
- Low-grade: 99.1°F to 100.4°F (37.3°C to 38°C)
- Moderate: 100.6°F to 102.2°F (38.1°C to 39°C)
- High-grade: 102.4°F to 105.8°F (39.1°C to 41°C)
Low-grade fevers often resolve on their own and may not need any treatment beyond rest and fluids. Moderate fevers are common with flu, COVID, and other infections. High-grade fevers deserve close attention, especially if they last more than a day or two.
Anything above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency that requires immediate treatment. At that level, the fever itself can cause organ damage.
What Counts as a Fever in Babies and Children
The same 100.4°F (38°C) threshold applies to children, but the stakes are higher with very young infants. The American Academy of Pediatrics focuses particular attention on babies between 8 and 60 days old who reach 100.4°F or above, because their immune systems are still developing and infections can progress quickly. For any infant under 3 months with a fever, getting a prompt medical evaluation is important.
Older children handle fevers much the way adults do. A moderate fever in a child who is still drinking fluids, making eye contact, and staying reasonably alert is usually less concerning than a lower fever in a child who is listless, refusing to drink, or difficult to wake.
Why Your Body Creates a Fever
Fever isn’t a malfunction. It’s a deliberate defense mechanism. When your immune system detects bacteria, viruses, or other invaders, it releases signaling molecules that act on a small region at the base of the brain. This region works like a thermostat, and those signals essentially turn the dial up.
Once the new target temperature is set higher, your body works to reach it. Blood vessels near the skin constrict to trap heat inside. You may start shivering, which generates heat through rapid muscle contractions. That’s why you can feel freezing cold even though your actual temperature is climbing. These processes continue until your blood temperature matches the new, higher set point. When the infection starts clearing, the set point drops back down, and you begin sweating to release the excess heat.
How Measurement Method Affects Your Reading
The number on your thermometer depends partly on where you take the reading. Rectal and ear temperatures run about 0.5°F to 1°F higher than an oral reading. Armpit and forehead temperatures run about 0.5°F to 1°F lower than oral. So a forehead reading of 99.5°F could be equivalent to an oral reading of 100°F or higher.
For infants, rectal measurement is considered the most accurate. For older children and adults, oral thermometers give reliable results as long as you haven’t just had something hot or cold to drink. Forehead and ear thermometers are convenient for quick checks but can be less precise. If you get a borderline reading with a forehead scanner and you’re feeling genuinely unwell, it’s worth confirming with an oral measurement.
Warning Signs That Accompany a Fever
The temperature number alone doesn’t tell the full story. A fever of 101°F with mild body aches is a very different situation from a fever of 101°F with a stiff neck, confusion, or a rash. Certain symptoms alongside a fever point to something more serious:
- Stiff neck with pain when bending the head forward: can signal meningitis
- Severe headache or unusual sensitivity to bright light
- Mental confusion, altered speech, or strange behavior
- Persistent vomiting
- Difficulty breathing or chest pain
- Rash that appears suddenly
- Seizures or convulsions
- Pain when urinating
In children, the behavioral cues matter as much as the thermometer. A child who seems confused, won’t make eye contact, or is unusually difficult to console warrants a call to their doctor, even if the fever number itself seems moderate. The combination of how high the fever is, how long it’s lasted, and what other symptoms are present gives a much clearer picture than any single number.