A fever in an adult is an oral temperature of 100.4°F (38°C) or higher. This is the threshold used by both the Mayo Clinic and the CDC. Your normal body temperature isn’t a fixed number; it fluctuates throughout the day, running lower in the morning and higher in the late afternoon, typically hovering somewhere around 98.6°F.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s your body deliberately raising its internal thermostat. A region deep in your brain acts as your central temperature control center, constantly monitoring and adjusting your body heat. When your immune system detects a threat, like a virus or bacterial infection, immune cells release signaling molecules that travel to this control center through your bloodstream and nervous system.
Once those signals arrive, your brain resets the target temperature higher. To reach that new set point, your body ramps up its metabolic rate, constricts blood vessels near the skin to trap heat inside, and reduces sweating. This is why you often feel chills at the start of a fever: your body is actively generating heat to reach a temperature it now considers “normal.” The elevated temperature helps your immune system work more efficiently and makes your body a less hospitable environment for many pathogens.
What Causes a Fever Beyond Infections
Most fevers in adults come from common infections like colds, the flu, urinary tract infections, or respiratory illnesses. But infection isn’t the only trigger. When a fever persists without an obvious cause, doctors investigate several other categories.
Cancers account for 20% to 30% of unexplained persistent fevers, particularly blood cancers like leukemia and lymphoma, as well as colorectal cancer. Autoimmune and inflammatory conditions, including lupus, rheumatoid arthritis, and Crohn’s disease, make up another 10% to 30% of cases. Certain medications can also cause drug-induced fevers, and blood clots, thyroid inflammation, and other miscellaneous conditions account for another 10% to 20%. These numbers come from cases where a fever lasts weeks without explanation, not from typical short-lived fevers.
How to Take an Accurate Temperature
The method you use matters. Oral thermometers are the most common choice for adults and provide good accuracy. Rectal readings are technically the most precise, but oral measurements are close enough for practical purposes and far more convenient.
Forehead thermometers are popular because they’re fast and non-invasive, but they can give misleading readings. Direct sunlight, cold air, and sweat on the forehead all throw them off. Ear thermometers face their own issues: earwax buildup, ear infections, and variations in ear canal shape can skew results. If you’re tracking a fever over time, stick with the same method each reading. There’s no reliable formula for converting between sites, so switching from forehead to oral mid-illness makes it hard to tell if your temperature is actually changing.
Fever Severity Levels
Not all fevers carry the same weight. A low-grade fever, generally between 100.4°F and 102°F, is common with routine viral infections and typically resolves on its own within a few days. It usually doesn’t require treatment unless it’s making you miserable.
A temperature of 103°F (39.4°C) or higher warrants a call to your doctor. At that point, the fever itself can start causing significant discomfort, including headache, muscle aches, and dehydration. Temperatures above 104°F to 105°F are considered dangerous and can affect organ function if sustained. Anything above 106°F is a medical emergency that requires immediate care.
Managing a Fever at Home
Most adult fevers don’t need aggressive treatment. Your body is using the fever as a defense mechanism, and a mild one will typically run its course in a few days. What matters most is staying comfortable and hydrated. Fever increases fluid loss through sweat and faster breathing, so drink more water than usual.
Over-the-counter options like acetaminophen and ibuprofen both reduce fever effectively. Acetaminophen should stay under 4,000 milligrams total in a 24-hour period to protect your liver. Combination products containing both medications are also available. Light clothing and a comfortable room temperature help more than bundling up under heavy blankets, which can trap heat and push your temperature higher. A lukewarm (not cold) bath can also bring mild relief.
Warning Signs That Need Immediate Attention
A fever on its own, even an uncomfortable one, is rarely dangerous for a healthy adult. What matters is what accompanies it. Seek immediate medical care if a fever comes with any of these:
- Confusion, altered speech, or strange behavior
- Seizures or convulsions
- Stiff neck, especially with pain when bending your head forward
- Difficulty breathing or chest pain
- Severe headache or unusual sensitivity to bright light
- Persistent vomiting
- Severe pain anywhere in the body
- A new or unexplained rash
- Pain when urinating or foul-smelling urine
These combinations can signal serious conditions like meningitis, sepsis, or other infections that need urgent treatment. A fever of 103°F or above, even without these additional symptoms, is worth a call to your healthcare provider to discuss next steps.