A fever indicates that your body’s immune system is actively responding to a threat, most often an infection. A temperature of 100.4°F (38°C) or higher, measured rectally or with an ear thermometer, is the standard threshold. While fevers feel unpleasant, they are one of your body’s most effective defense mechanisms, and understanding what triggers them can help you figure out whether yours needs attention or just time.
How Your Body Creates a Fever
A fever isn’t your body malfunctioning. It’s a deliberate, coordinated response. When your immune cells detect a pathogen or inflammatory trigger, they release signaling molecules called cytokines. These cytokines travel to the hypothalamus, the part of your brain that acts as your internal thermostat, and essentially tell it to turn up the heat. Your brain then raises its temperature “set point,” the same way you’d crank up a thermostat at home. Your body responds by constricting blood vessels near the skin (making you feel cold and shivery) and ramping up heat production until your core temperature reaches the new, higher target.
Once the threat is resolved, the set point drops back to normal, and you start sweating as your body works to cool itself down. That breaking-a-fever sensation is literally your thermostat resetting.
Why Fever Helps You Fight Infection
The elevated temperature serves two purposes at once: it weakens pathogens and strengthens your immune response. Many bacteria, viruses, and especially fungi grow best within a narrow temperature range. Raising your body temperature even a degree or two can push these invaders out of their comfort zone, slowing their ability to replicate. Fungal infections are a striking example: most fungi thrive at temperatures below normal mammalian body heat, so a fever makes the environment even more hostile to them.
At the same time, a fever supercharges your immune system. Higher temperatures improve the ability of white blood cells to travel through your bloodstream and reach infected tissue. They also enhance your body’s ability to present pieces of invading organisms to other immune cells, speeding up antibody production and helping T cells (the immune cells that coordinate targeted attacks) work more efficiently. In short, fever is your body running a faster, more aggressive defense.
Infections: The Most Common Cause
The vast majority of fevers come from infections. Viral illnesses like the flu, COVID-19, and common colds are the most frequent triggers. Bacterial infections, including strep throat, urinary tract infections, sinus infections, and pneumonia, also commonly cause fevers. In cases where a fever persists without an obvious source, infections still account for 20% to 40% of diagnoses, with causes ranging from dental abscesses to tuberculosis.
The pattern of your fever can sometimes hint at its cause. A fever that spikes and drops repeatedly over days might suggest a different type of infection than one that stays consistently elevated. But fever patterns alone are rarely enough to pinpoint a diagnosis.
Non-Infectious Causes
Infections aren’t the only explanation. A fever can also signal that your immune system is misfiring rather than fighting off a genuine threat. Autoimmune and inflammatory conditions account for 10% to 30% of unexplained fevers. Conditions like lupus, rheumatoid arthritis, and Crohn’s disease can all trigger fevers because they involve chronic immune activation. Inflammatory blood vessel conditions like giant cell arteritis are another recognized cause.
Certain medications can also cause fever as a side effect. Anticonvulsants, some antibiotics, and blood thinners are among the more common culprits. Drug-related fevers can occur through allergic-type reactions or through rarer but more dangerous responses like neuroleptic malignant syndrome (a reaction to certain psychiatric medications) or serotonin syndrome. Cancer is another non-infectious cause, particularly lymphomas and leukemias, where the tumor itself or the body’s reaction to it produces fever-triggering signals.
Thyroid inflammation, blood clots, and even heat stroke can raise your temperature as well, though heat stroke technically raises body temperature through a different mechanism than true fever, since it overwhelms your thermostat rather than resetting it.
What Temperature Actually Counts as a Fever
Normal body temperature varies throughout the day and from person to person, but the widely accepted fever threshold is 100.4°F (38°C) when measured rectally, with an ear thermometer, or at the temple. An oral temperature of 100°F (37.8°C) or higher also qualifies. Armpit readings run lower, so 99°F (37.2°C) in the armpit is considered a fever.
Rectal readings are the most accurate, which is why they’re the standard for infants. Oral thermometers provide similar accuracy and are more practical for older children and adults. Regardless of the method, consistency matters more than converting between types. If you’re tracking a fever over time, take the temperature the same way each time for the most reliable comparison.
Fever in Infants and Young Children
Fever in babies follows stricter rules because their immune systems are immature and infections can progress quickly. Any fever in a baby younger than 3 months old warrants a call to a healthcare provider, regardless of how the baby seems otherwise. For babies 3 to 6 months old, a temperature of 100.4°F or higher, or any fever paired with signs of illness, should prompt a call. Between 6 and 24 months, a fever above 100.4°F that lasts more than a day deserves medical attention.
For children of any age, a fever lasting more than three days should be evaluated. A seizure triggered by fever (febrile seizure) is frightening but usually not dangerous on its own. If a seizure lasts more than five minutes or the child doesn’t recover quickly, call 911.
Warning Signs That Accompany a Fever
Most fevers resolve on their own within a few days and simply indicate that your body is doing its job. But certain symptoms alongside a fever point to something more serious. Seek immediate medical attention if a fever comes with any of the following:
- Stiff neck with pain when bending the head forward, which can suggest meningitis
- Mental confusion, altered speech, or strange behavior
- Severe headache or unusual sensitivity to bright light
- A new rash, particularly one that doesn’t fade when pressed
- Persistent vomiting
- Difficulty breathing or chest pain
- Abdominal pain or pain when urinating
- Seizures or convulsions
In children, look for listlessness, poor eye contact, extreme irritability, or repeated vomiting. A child who develops a fever after being left in a hot car needs emergency care immediately.
When a Fever Doesn’t Need Treatment
A low-grade fever in an otherwise healthy adult or older child is typically your immune system working as designed. Bringing it down with over-the-counter fever reducers can make you more comfortable, but it doesn’t speed up recovery from the underlying illness. Some evidence suggests that letting a mild fever run its course may actually help you clear infections faster, given the immune-boosting effects of elevated temperature.
That said, comfort matters. If a fever is making you miserable, unable to sleep, or unable to stay hydrated, treating it is perfectly reasonable. The goal isn’t to hit a specific number on the thermometer. It’s to support your body while it handles whatever triggered the fever in the first place.