ConditionInfections
Fever (Pyrexia)
A fever, or pyrexia, is a temperature of 38 °C (100.4 °F) or higher caused by the brain deliberately raising the body's thermostat, usually in response to infection. It is a defense mechanism: higher temperatures slow the growth of many germs and speed up immune responses. Fever is different from hyperthermia (overheating), such as heat stroke, where the thermostat is normal but the body cannot shed heat. Most fevers are safe and self-limiting; the cause matters more than the number.
Also called: Pyrexia, febrile illness, high temperature, febrile response
At a glance
- Definition
- Core temperature of 38 °C (100.4 °F) or higher from a raised hypothalamic set point.
- Usual cause
- Infection, mostly viral; also inflammation, medicines, cancer and blood clots.
- Stages
- Chills as temperature rises, a hot plateau, then sweating as it falls.
- Measuring
- Rectal in babies; oral, ear or forehead in older children and adults.
- Treatment
- Treat the cause; fever reducers for comfort, not to reach a number.
- Complications
- Dehydration and, in young children, febrile seizures; usually not brain damage.
When to get help
Call 911 or go to the ER
- A baby under 3 months with a temperature of 38 °C (100.4 °F) or higher
- Fever with a non-blanching rash, stiff neck, confusion or extreme drowsiness
- Fever with labored breathing, mottled or bluish skin, or no urine for 12 to 24 hours
- Temperature of 40 °C (104 °F) or more with hot, dry skin and confusion after heat exposure (heat stroke)
- A seizure lasting more than 5 minutes
See a doctor within a day or two
- Fever in someone with a weakened immune system, on chemotherapy, or without a spleen
- Fever after travel to a malaria region
- Fever lasting more than 3 days in adults or 5 days in children
- A first febrile seizure in a child, even if brief
Book a routine appointment
- Repeated fevers with no clear cause over weeks
- Low-grade fevers with night sweats and weight loss
- Fevers that recur on a regular schedule
Usually safe to manage at home
- Fever below 39 °C (102.2 °F) with common cold or flu symptoms in a healthy, alert child or adult who is drinking well
How fever works
Body temperature is regulated by the hypothalamus, a small area at the base of the brain that acts like a thermostat. During infection, immune cells release signaling molecules called pyrogens, such as interleukin-1, interleukin-6 and tumor necrosis factor. These trigger production of prostaglandin E2 in the hypothalamus, which raises the set point.
The body then works to reach the new, higher temperature by:
- narrowing blood vessels in the skin to conserve heat (cold hands and feet)
- shivering to generate heat
- behavior changes such as seeking blankets and curling up
Medicines such as acetaminophen (paracetamol) and ibuprofen lower fever by blocking prostaglandin production, which resets the thermostat.
Why fever helps
Many bacteria and viruses replicate less efficiently at higher temperatures. Fever also boosts white blood cell activity and antibody production. This is why fever is not, in itself, the enemy. Treating it does not make infections last longer in a clinically meaningful way, but there is no need to suppress it if the person is comfortable.
The stages of fever
| Stage | What happens | How it feels |
|---|---|---|
| Rising phase (chill) | Set point raised; body generates and conserves heat | Cold, shivering, goosebumps, pale skin |
| Plateau | Temperature reaches the new set point | Hot, flushed, tired, aching |
| Falling phase (defervescence) | Set point returns to normal; body sheds heat | Sweating, flushed skin, relief |
Severe shaking chills (rigors) suggest a rapid rise and are common with bacterial infections such as kidney infection and pneumonia, as well as flu and malaria.
Fever vs hyperthermia
| Fever | Hyperthermia | |
|---|---|---|
| Thermostat | Raised by pyrogens | Normal |
| Cause | Infection, inflammation | Heat exposure, exertion, some drugs |
| Skin | Often shivering, then sweating | Hot; may be dry in heat stroke |
| Responds to fever reducers | Yes | No |
| Treatment | Treat cause; medicines for comfort | Rapid physical cooling |
Heat stroke is a medical emergency: rapid cooling, not medicine, is the treatment.
Measuring temperature correctly
- Babies under 3 months: a digital rectal thermometer is most accurate; many guidelines accept an armpit reading as a screen.
- Children 3 months to 4 years: digital armpit, ear (from 6 months) or temporal artery (forehead) thermometers.
- Older children and adults: oral digital thermometers are accurate. Wait 15 to 30 minutes after eating or drinking.
- Avoid mercury thermometers and strip thermometers. Touching the forehead is unreliable.
Causes
Infections
Most acute fevers are viral (colds, flu, COVID-19, gastroenteritis, roseola) or bacterial (ear infections, strep throat, urinary infections, pneumonia, skin infections). See fever in children for age-specific causes and care.
Other causes
- Inflammatory diseases: rheumatoid arthritis, lupus, vasculitis, inflammatory bowel disease, Still's disease.
- Drug fever: a reaction to antibiotics, anti-seizure medicines and others.
- Cancers: lymphoma, leukemia, kidney cancer.
- Blood clots: deep vein thrombosis and pulmonary embolism can cause low-grade fever.
- Periodic fever syndromes: inherited conditions with recurring fevers on a pattern.
- Hormonal: an overactive thyroid crisis (thyroid storm).
- Brain injury affecting the hypothalamus.
Fever of unknown origin
Doctors use this term for a fever of 38.3 °C (101 °F) or more on several occasions over at least 3 weeks without a diagnosis after initial tests. The main categories are hidden infections (such as tuberculosis, endocarditis or abscesses), cancers, and inflammatory diseases. Investigation may include repeated blood cultures, CT or PET scans, echocardiography and biopsies.
Diagnosis
The goal is to find the cause. A doctor considers age, other symptoms, exposures, travel, vaccinations, medicines and immune status, then examines the person carefully. Tests are chosen according to suspicion and severity:
- urine test for infection
- blood count, CRP and procalcitonin (markers of inflammation and bacterial infection)
- blood cultures
- swabs for flu, COVID-19, strep throat
- chest X-ray
- lumbar puncture in suspected meningitis
- malaria tests after travel
Treatment
- Treat the cause: antibiotics for bacterial infections; antivirals for influenza in high-risk people; stopping a culprit medicine.
- Comfort measures: fluids, rest and light clothing.
- Fever reducers: acetaminophen or ibuprofen, at the label dose by weight for children, mainly for comfort. Do not alternate or combine them routinely unless a clinician advises it. Never give aspirin to children or teenagers because of the risk of Reye syndrome. Avoid ibuprofen with kidney disease, stomach ulcers or dehydration.
- Avoid cold baths, ice and alcohol rubs, which cause shivering and discomfort.
Complications
- Dehydration, especially in babies and older adults.
- Febrile seizures: affect about 2 to 5% of children between 6 months and 5 years. They are frightening but usually brief and harmless. Fever-reducing medicine does not prevent them.
- Worsening of chronic conditions, such as heart failure, in older adults.
- Delirium in older people.
- Sepsis: not a complication of fever itself, but of the underlying infection. Watch for its warning signs.
Fevers from infection almost never rise high enough to damage the brain. Temperatures above 41 to 42 °C (105.8 to 107.6 °F) usually reflect hyperthermia rather than fever.
Frequently asked questions
How does a fever fight infection?
Fever slows the reproduction of many bacteria and viruses and speeds up immune activity, including white blood cell movement and antibody production. It is a deliberate defense mechanism controlled by the brain, not a malfunction.
What are the stages of a fever?
There are three: a rising phase with chills and shivering as the body heats up, a plateau where you feel hot and flushed, and a falling phase with sweating as the temperature returns to normal.
Is a high fever dangerous?
Usually not in itself. Fevers from infection rarely go above 41 °C (105.8 °F) and do not damage the brain. The danger lies in the cause, such as meningitis or sepsis, and in dehydration. How unwell the person looks matters more than the number.
What is the difference between fever and hyperthermia?
In fever, the brain raises the body's thermostat in response to infection or inflammation. In hyperthermia, such as heat stroke, the thermostat is normal but the body cannot get rid of heat. Fever reducers work for fever but not for hyperthermia, which needs rapid cooling.
What is fever of unknown origin?
It means repeated fevers of 38.3 °C (101 °F) or higher lasting at least 3 weeks without a diagnosis after standard tests. Hidden infections, cancers such as lymphoma, and inflammatory diseases are the main causes. Specialist investigation usually finds the cause.
Sources
This guide was written from current patient and clinical guidance, including:
- MedlinePlus: Fever
- NICE: Fever in under 5s, assessment and initial management
- American Academy of Pediatrics: Fever and antipyretic use in children (clinical report)
- NHS: High temperature (fever) in children
- Merck Manual Professional: Fever in adults
- CDC: Sepsis, signs and symptoms