A fever in adults is a body temperature at or above 100.4°F (38°C). Normal body temperature hovers around 98.6°F (37°C), though it naturally fluctuates throughout the day, running slightly lower in the morning and higher in the late afternoon. A fever itself isn’t an illness. It’s your immune system’s response to a threat, raising your internal temperature to make the body a less hospitable environment for viruses and bacteria.
How Fever Is Measured
The 100.4°F threshold applies whether you’re measuring orally, rectally, or with an ear (tympanic) thermometer. Forehead and no-contact infrared thermometers, which became widespread during the COVID-19 pandemic, are just as accurate as oral or rectal thermometers when used correctly. That said, internal readings (oral or rectal) remain the gold standard in clinical settings because they’re less affected by environmental factors like ambient temperature or sweating.
If you’re using a forehead thermometer at home, keep in mind that readings can skew low if you’ve just come in from cold weather or if you’re sweating heavily. For the most reliable result, sit indoors for a few minutes before scanning.
What Causes a Fever
Infections are the most common trigger. Your body detects bacteria, viruses, or other pathogens and releases chemical signals that reset your internal thermostat higher. This is why fevers so often accompany colds, the flu, urinary tract infections, and pneumonia.
But infection isn’t the only cause. Autoimmune and inflammatory conditions like lupus, rheumatoid arthritis, and sarcoidosis can produce recurring fevers because the immune system is chronically activated even without an outside invader. Certain cancers, particularly lymphoma, leukemia, and kidney cancer, are known to cause persistent, unexplained fevers. Medications can also be responsible. Drugs used for gout, blood pressure, seizures, and even some common antihistamines and diuretics occasionally trigger what’s known as drug fever, which resolves once the medication is stopped.
A fever that lasts more than a few weeks without an obvious explanation falls into a category called “fever of unknown origin,” which typically requires more extensive testing to pin down.
Low-Grade vs. High Fever
Not all fevers carry the same level of concern. A temperature between 100.4°F and 102°F is generally considered low-grade. It’s common with mild infections and often resolves on its own within a day or two. You may feel achy, tired, and slightly chilled, but your body is doing what it’s supposed to do.
Temperatures between 102°F and 104°F signal a more significant immune response. You’ll likely feel noticeably unwell, with chills, sweating, headache, and muscle pain. This range warrants closer attention, especially if the fever persists for more than two or three days.
A fever above 104°F (40°C) is considered high and calls for a conversation with your doctor. Above 106.7°F (41.5°C), the situation becomes a medical emergency called hyperpyrexia, which can cause organ damage if not treated quickly. This is rare and usually associated with severe infections, drug reactions, or conditions affecting the brain’s temperature-regulating center.
Fever vs. Hyperthermia
Fever and hyperthermia both involve elevated body temperature, but they work through completely different mechanisms. With a fever, your brain deliberately raises its temperature set point as part of an immune response. With hyperthermia, your body simply can’t cool itself down. Heatstroke is the classic example: the external heat overwhelms your cooling systems, and your temperature climbs dangerously. This distinction matters because standard fever-reducing medications won’t help with hyperthermia. Cooling the body externally is what’s needed.
Managing a Fever at Home
Most adult fevers don’t need aggressive treatment. A low-grade fever from a common virus is your immune system working, and letting it run its course can actually help you recover faster. The goal of home treatment is comfort, not eliminating the fever entirely.
Over-the-counter pain relievers like acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) both reduce fever effectively. Acetaminophen should stay under 4,000 milligrams total in a 24-hour period to protect your liver. If you’re taking a combination product that contains both acetaminophen and ibuprofen, follow the package directions carefully, as it’s easy to accidentally double up on acetaminophen if you’re also taking other cold or flu medications that contain it.
Beyond medication, stay hydrated. Fever increases fluid loss through sweating and faster breathing, so water, broth, and electrolyte drinks help more than you might think. Dress in light layers, keep the room comfortable, and rest. Avoid ice baths or alcohol rubs, which can cause shivering and actually raise your core temperature further.
When a Fever Signals Something Serious
A fever on its own, even a moderately high one, is usually manageable. What changes the picture is the company it keeps. Seek immediate medical attention if a fever occurs alongside 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 area
- Painful urination or foul-smelling urine, suggesting a worsening urinary infection
A fever above 104°F on its own, even without these additional symptoms, is worth a call to your doctor. The same applies to any fever that persists for more than three days without improvement, or one that goes away and then returns. These patterns can point to a secondary infection or an underlying condition that needs attention beyond what rest and fluids can provide.