A fever happens when your brain’s internal thermostat, located in a region called the hypothalamus, deliberately raises your body’s temperature set point above its normal range of 97.7°F to 99.5°F. A temperature of 100.4°F (38°C) or higher, measured rectally or with an ear thermometer, is generally considered a fever. The most common reason you got one is that your immune system detected something it wants to fight, usually a virus or bacterium, and triggered a chain reaction that told your brain to turn up the heat.
What Happens Inside Your Body
When your immune system encounters an invader like a virus, bacteria, or fungus, it releases signaling molecules called cytokines. Three of the most important ones for fever are IL-1, IL-6, and TNF. These molecules do two things: they rally more immune cells to join the fight, and they kick off a chemical process that ultimately produces prostaglandins, the molecules that act directly on your hypothalamus.
Those prostaglandins essentially reset your thermostat. If your normal set point is 98.6°F, the hypothalamus might bump it to 101°F or 102°F. Your brain now treats your actual body temperature as “too cold,” even though it was perfectly normal five minutes ago. This is why common fever-reducing medications work: they block the enzyme (COX-2) that produces prostaglandins, which lowers the set point back to normal.
There are two routes these fever signals travel. One is through the bloodstream, where immune cells in the blood and liver send prostaglandins directly to the brain. The other is through nerve fibers, particularly the vagus nerve, which carries the alarm signal from your gut and organs up to the brain. Both pathways lead to the same result: your hypothalamus decides to raise your temperature.
Why Your Body Bothers With a Fever
A fever isn’t a malfunction. It’s a deliberate defense strategy. Higher body temperatures make it harder for many viruses and bacteria to reproduce, and they speed up certain immune responses. Your body is essentially making itself a less hospitable environment for whatever is trying to infect you. That said, the fever itself can make you feel terrible, which is why it often makes sense to manage the symptoms even though the mechanism is protective.
The Three Stages You Feel
A fever typically moves through three distinct phases, and recognizing them can help you understand where you are in the process.
The Chill Phase
This is the confusing part. Your thermostat has just been raised, so your brain perceives your current (normal) temperature as too low. You feel cold, even if the room is warm. Your body starts shivering to generate heat, and blood vessels near your skin constrict to reduce heat loss. You might pile on blankets and still feel like you can’t get warm. Your temperature is actually climbing during this phase, not dropping.
The Plateau Phase
Once your core temperature reaches the new set point, the shivering stops. Now you feel hot, flushed, and thirsty. Your skin may look red. This is when your temperature stays relatively stable at its elevated level, and your immune system is working at peak intensity. This phase can last hours or days depending on the cause.
The Sweating Phase
When your immune system starts gaining the upper hand, or when you take a fever reducer, the hypothalamus lowers the set point back toward normal. Now your actual temperature is higher than the target, so your body does the opposite of what it did in phase one: blood vessels dilate, and you sweat profusely. The evaporation cools you down, and your temperature gradually returns to baseline around 98.6°F.
The Most Common Causes
Infections are by far the most frequent reason for a fever. Upper respiratory viruses (common colds, flu, COVID-19), stomach bugs, urinary tract infections, sinus infections, and ear infections are the usual suspects. Most fevers you’ll experience in your lifetime come from a viral infection that will resolve on its own within a few days.
Bacterial infections can also cause fevers, and they tend to produce higher, more persistent ones. Strep throat, pneumonia, kidney infections, and skin infections like cellulitis all commonly trigger fevers. Fungal infections are less common but do occur, particularly in certain geographic regions or in people with weakened immune systems.
But infections aren’t the only possibility. Several non-infectious conditions trigger the same cytokine cascade:
- Autoimmune and inflammatory conditions like rheumatoid arthritis, lupus, gout flares, and inflammatory bowel disease can cause fevers because the immune system is attacking the body’s own tissues and releasing the same fever-triggering cytokines.
- Vaccines can cause a low-grade fever for a day or two. This is a sign your immune system is responding to the vaccine, not a sign of illness.
- Some medications can trigger fevers as a side effect, sometimes called drug fever. If a fever starts shortly after beginning a new medication, that connection is worth noting.
- Certain cancers, particularly lymphomas and leukemias, sometimes produce persistent or recurring fevers.
Fever vs. Overheating
Not every high temperature is a fever. Hyperthermia, which includes heat exhaustion and heatstroke, raises your body temperature without changing the hypothalamic set point. The distinction matters. In a fever, your brain is in control, deliberately raising the temperature and maintaining it within a range. In heatstroke, your body has simply absorbed or generated more heat than it can shed, and the temperature rises in an uncontrolled way. Heatstroke is a medical emergency because the body has lost its ability to regulate temperature. Fever almost never reaches those dangerous levels on its own.
Getting an Accurate Reading
How you take your temperature affects the number you see. Rectal readings are the most accurate, followed closely by oral readings. Ear thermometers work well for most people but aren’t recommended for infants under 7 months, and earwax or ear infections can throw off the reading. Contactless forehead thermometers are convenient but less reliable, especially if you’re sweaty, outdoors, or holding the device too far from the skin.
Temperatures vary slightly depending on where you measure. An armpit reading of 99°F, an oral reading of 100°F, and a rectal reading of 100.4°F all roughly indicate a fever. Rather than trying to convert between methods, pick one and use it consistently so you can track changes over time.
When a Fever Signals Something Serious
Most fevers are harmless and resolve within a few days. But certain combinations of symptoms suggest something more dangerous, like meningitis, which is an infection of the membranes around the brain and spinal cord. A fever paired with a severe headache, stiff neck, confusion, sensitivity to light, vomiting, or seizures needs immediate medical attention.
In infants, the warning signs look different: a high fever with constant crying, extreme sleepiness or irritability, poor feeding, vomiting, or a bulging soft spot on the head. Any fever of 100.4°F or higher in a baby under 3 months old warrants a call to a healthcare provider regardless of other symptoms.
When Doctors Can’t Find the Cause
Sometimes a fever persists and no obvious source turns up. If a temperature of 101°F or higher is documented on multiple occasions and initial testing doesn’t reveal an answer, doctors classify it as a fever of unknown origin. The workup typically starts with a thorough history and physical exam, blood tests, urine cultures, blood cultures, and a chest X-ray. If those come back normal, additional testing might include screening for viruses like Epstein-Barr or HIV, tuberculosis testing, or imaging of the abdomen and pelvis to look for hidden infections or abscesses. About a third of fever-of-unknown-origin cases turn out to be infections that were just hard to find, another third involve inflammatory or autoimmune conditions, and a smaller percentage involve cancer.