A fever in adults is an oral temperature of 100.4°F (38°C) or higher. That number is the standard threshold used by the CDC, Mayo Clinic, and most clinical guidelines. Your normal resting body temperature sits around 98.6°F, though it naturally fluctuates throughout the day and varies from person to person.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s your body deliberately raising its internal thermostat in response to a threat, usually an infection. A small region deep in your brain acts as that thermostat, maintaining a temperature set point. When your immune system detects bacteria, viruses, or other invaders, it releases signaling molecules that push that set point higher.
Once the new, higher set point is established, your body works to reach it. Blood vessels near your skin constrict to trap heat inside. You may start shivering, which generates warmth through rapid muscle contractions. This is why you can feel freezing cold even though your temperature is climbing. The chills continue until your blood temperature matches the new set point. When the infection clears or you take a fever reducer, the set point drops back down, and you start sweating to release the excess heat.
Temperature Varies by Where You Measure
The 100.4°F threshold applies to oral, rectal, and ear measurements. But if you’re taking your temperature under your arm (axillary), the reading runs lower. An armpit temperature of 99°F (37.2°C) or higher suggests a fever. Armpit readings are the least accurate of the common methods, so if yours is borderline, it’s worth confirming with an oral reading.
Rectal temperatures tend to read about 0.5 to 1°F higher than oral, while ear (tympanic) thermometers generally align closely with oral readings when used correctly. The method matters less than being consistent. If you’re tracking a fever over hours or days, use the same thermometer in the same location each time.
What Counts as a Fever in Older Adults
Older adults run cooler than younger people. Data from North Carolina nursing homes found the average baseline temperature in residents was 97.7°F, a full degree below the traditional “normal” of 98.6°F. This means an older adult can have a clinically meaningful fever at temperatures that look unremarkable on paper.
The most reliable definition for this group is a temperature 1.4°F above that person’s known baseline. If you don’t know the baseline, a reading of 99°F or higher is a reasonable rule of thumb. This is especially important because older adults with serious infections sometimes never hit 100.4°F, and waiting for that number can delay treatment.
Low-Grade vs. High Fevers
Not all fevers carry the same weight. A temperature between 100.4°F and 102°F is generally considered low-grade. These fevers are common with routine viral infections like colds and flu, and they often resolve on their own within a few days. Most healthy adults tolerate low-grade fevers without any complications.
Temperatures between 102°F and 104°F signal a more significant immune response. You’ll likely feel noticeably unwell, with body aches, fatigue, and sometimes headache. Above 104°F (40°C) is considered a high fever and warrants prompt medical attention. Temperatures above 106°F (41.1°C) are rare but dangerous, as sustained heat at that level can damage organs.
When a Fever Needs Medical Attention
A fever by itself is rarely dangerous in otherwise healthy adults. It becomes concerning based on how high it goes, how long it lasts, and what other symptoms accompany it. A fever lasting more than three days, or one that keeps climbing despite home care, is worth a call to your doctor.
Certain symptoms alongside a fever signal something more serious. Seek immediate medical help if you have a fever with any of the following:
- Confusion or loss of consciousness
- Seizure
- Stiff neck
- Trouble breathing
- Severe pain anywhere in the body
- Swelling or inflammation that’s getting worse
- Painful urination or foul-smelling urine
These combinations can indicate infections like meningitis, sepsis, or kidney infections that need treatment quickly.
Managing a Fever at Home
Most fevers don’t need to be treated aggressively. Since fever is part of your immune response, bringing it down isn’t always necessary if you’re otherwise comfortable. The main reasons to take a fever reducer are discomfort, poor sleep, or a temperature climbing above 102°F.
Over-the-counter options include acetaminophen and ibuprofen. Both are effective at lowering temperature and relieving the aches that come with it. Follow the dosing instructions on the package and avoid exceeding 4,000 milligrams of acetaminophen in a 24-hour period, as higher amounts can cause liver damage. If your symptoms haven’t improved after 10 days, or they’re getting worse, that’s a sign you need a professional evaluation.
Staying hydrated matters more than most people realize. Fever increases fluid loss through sweat and faster breathing. Water, broth, and electrolyte drinks help prevent dehydration, which can make you feel significantly worse on top of the fever itself. Light clothing and a comfortable room temperature also help your body regulate heat more effectively than bundling up under heavy blankets.