A fever in adults is a body temperature above 100.4°F (38°C). This threshold applies regardless of how you measure it, though readings vary slightly depending on the method. Fever itself isn’t a disease. It’s your body’s deliberate response to an infection or illness, driven by a temporary reset of your internal thermostat.
How Your Body Creates a Fever
Your brain has a built-in thermostat located in a region called the hypothalamus. Normally, it keeps your core temperature hovering around 98.6°F, though healthy people can run slightly above or below that number throughout the day. When your immune system detects an invader, it releases chemical signals that tell the hypothalamus to raise its set point, essentially reprogramming your thermostat to a higher target temperature.
Once that new set point is established, your body treats its current normal temperature as “too cold” and activates the same warming mechanisms you’d use on a freezing day. Blood vessels near your skin constrict to trap heat inside. Your metabolism ramps up to generate more warmth. If the gap between your actual temperature and the new set point is large enough, you start shivering, which is your muscles rapidly contracting to produce heat. This is why you can feel freezing cold even while running a fever of 102°F. Your body is genuinely trying to warm itself up to match the new target.
What a Fever Feels Like
The rising phase of a fever typically brings chills, shivering, and an urge to pile on blankets. You may also notice muscle aches, general weakness, and a loss of appetite. Once your body reaches its new set point, the chills usually subside, replaced by a feeling of being flushed and warm. When the fever breaks, meaning the hypothalamus resets back to normal, your body needs to shed all that extra heat quickly. This is the sweating phase, and it can be dramatic enough to soak through clothing or sheets.
Headaches are common during a fever, partly because of dehydration and partly because of the inflammatory signals circulating through your body. Some people also experience irritability, difficulty concentrating, or a general foggy feeling. These symptoms are all side effects of the immune response rather than signs of something separate going wrong.
Temperature Ranges and Severity
Not all fevers carry the same level of concern. A reading between 100.4°F and 102°F is generally considered a low-grade fever, common with mild viral infections like colds. Temperatures from 102°F to 104°F represent a more significant fever that warrants closer attention, especially if it persists for more than a day or two. Anything above 104°F (40°C) is high enough that you should call a doctor.
At the extreme end, a body temperature above 106.7°F (41.5°C) is classified as hyperpyrexia, a medical emergency that can damage organs and requires immediate treatment. This level of fever is rare and typically associated with severe infections, drug reactions, or heat stroke rather than a common cold or flu.
How to Get an Accurate Reading
Rectal thermometers provide the most accurate reading, but oral thermometers offer similar accuracy and are far more practical for adults. Ear thermometers are convenient but can give unreliable results if earwax is present, if you have an ear infection, or if the probe isn’t angled correctly. Forehead thermometers are the least invasive option, though they tend to be less accurate, particularly in direct sunlight, cold environments, or when the skin is sweaty.
One important point: there’s no reliable formula for converting between measurement sites. You can’t simply add a degree to an armpit reading and treat it as an oral temperature. The best approach is to use the same method consistently so you can track changes over time rather than comparing readings taken from different parts of your body.
Common Causes in Adults
The vast majority of fevers in adults are caused by infections. Viral illnesses like the flu, COVID-19, and the common cold are the most frequent culprits, and these fevers typically last anywhere from three to seven days before resolving on their own. Bacterial infections, such as urinary tract infections, strep throat, or pneumonia, also cause fevers but often produce more localized symptoms alongside the elevated temperature.
One pattern worth knowing: a viral illness that seems to be improving and then suddenly worsens, with a new spike in fever and fresh symptoms, can signal a secondary bacterial infection that may need antibiotics. This is different from a fever that simply lingers at the same level for several days.
Less common causes include autoimmune conditions, certain medications, and heat-related illness. Fever from heat exposure (heat stroke) works through a different mechanism than infection-driven fever. In heat stroke, the body’s cooling system fails rather than the thermostat being deliberately raised, and it requires a different treatment approach.
When a Fever Needs Medical Attention
A fever above 104°F warrants a call to your doctor. Below that threshold, the decision depends more on accompanying symptoms and duration than the number on the thermometer. Seek immediate medical help if a fever comes with any of the following:
- Seizure or loss of consciousness
- Confusion or difficulty staying alert
- Stiff neck, which can indicate meningitis
- Trouble breathing
- Severe pain anywhere in the body
- Swelling or inflammation in any body part
- Pain during urination or foul-smelling urine
A persistent low-grade fever that lasts more than five days without improving also deserves evaluation, even if you feel otherwise stable. The same applies to a fever that gets better and then returns, as this pattern can indicate a new or worsening infection.
Managing a Fever at Home
For most adults with a mild to moderate fever, the priority is staying hydrated and resting. Fever increases your metabolic rate, which means you lose fluids faster than usual. Water, broth, and electrolyte drinks all help. Avoid alcohol and caffeine, both of which can worsen dehydration.
Over-the-counter fever reducers can bring your temperature down and relieve the muscle aches and headaches that come with it. Keep in mind that lowering a fever doesn’t slow your recovery. You’re treating comfort, not fighting the underlying illness. If a fever is mild and you feel reasonably functional, there’s no medical requirement to reduce it. Your immune system is doing what it’s designed to do.
Avoid bundling up in heavy blankets during the hot phase of a fever, as this traps heat and can push your temperature higher. Lightweight clothing and a comfortable room temperature are more helpful than ice baths or cold compresses, which can trigger shivering and paradoxically raise your core temperature further.