SymptomWhole body & general
Fever: What Counts as a Fever, Causes and When to Worry
A fever is a body temperature of 38 °C (100.4 °F) or higher, usually the immune system's response to an infection such as a cold, flu, COVID-19 or a urinary infection. Most fevers last 2 to 3 days and are not dangerous in themselves. Babies under 3 months with any fever, and anyone with a fever plus a stiff neck, a rash that does not fade, confusion or trouble breathing, need a doctor straight away.
Also called: High temperature, pyrexia, febrile illness, running a temperature, feeling feverish
At a glance
- Fever threshold
- 38 °C (100.4 °F) or higher, measured with a thermometer.
- Most common causes
- Viral infections (colds, flu, COVID-19), urinary infections, chest infections, ear and throat infections.
- Measuring
- Rectal in babies; oral, ear or forehead in older children and adults. A hand on the forehead is not reliable.
- Typical duration
- Viral fevers usually last 2 to 3 days, sometimes up to 5.
- When to worry
- Any fever in a baby under 3 months, fever over 3 days, or fever with stiff neck, rash, confusion or breathing trouble.
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 stiff neck, severe headache, confusion or a purple or red rash that does not fade when a glass is pressed on it
- Fever with difficulty breathing, blue or gray lips, or chest pain
- A seizure, or a child who is floppy, hard to wake or not responding normally
- Fever with signs of sepsis: shivering, mottled or cold skin, very fast breathing, no urine for a day, or feeling like you might die
See a doctor within a day or two
- A baby aged 3 to 6 months with a temperature of 39 °C (102.2 °F) or higher
- Fever above 39.4 °C (103 °F) in an adult that does not come down with medicine
- Fever with pain on passing urine, back or side pain, or severe sore throat with trouble swallowing
- Fever after recent travel abroad, especially to a malaria area
- Fever if you are on chemotherapy, take immune-suppressing medicines or have had your spleen removed
Book a routine appointment
- Fever lasting more than 3 days in an adult or 5 days in a child
- A child with a fever who is drinking less than half the usual amount
- Repeated night sweats or low-grade fevers for more than 2 weeks
Usually safe to manage at home
- A fever under 39 °C (102.2 °F) with cold or flu symptoms in an otherwise healthy adult or child who is drinking and alert
- A low-grade fever of up to 38.5 °C (101.3 °F) for a day or two after a vaccine
What temperature is a fever?
Normal body temperature averages about 37 °C (98.6 °F) but varies by 0.5 °C (about 1 °F) or so during the day. It is lowest in the early morning and highest in the late afternoon.
| Reading | What it means |
|---|---|
| 36.1 to 37.2 °C (97 to 99 °F) | Normal range |
| 37.3 to 37.9 °C (99.1 to 100.3 °F) | Slightly raised, not a fever by most definitions |
| 38 °C (100.4 °F) or higher | Fever |
| 39.4 °C (103 °F) or higher | High fever in adults |
| 41 °C (105.8 °F) or higher | Very high; seek urgent care |
Where you measure matters (see the thermometer table below). Feeling a forehead with your hand is not accurate: skin can feel hot from a warm room or flushing when the core temperature is normal.
Common causes of fever
Infections (most fevers)
- Viral infections: colds, flu, COVID-19, stomach bugs and childhood illnesses such as roseola and chickenpox. Most clear on their own.
- Ear, throat and sinus infections: ear pain, a very sore throat with no cough, or facial pain.
- Urinary tract infections: burning when passing urine and frequency. A kidney infection adds high fever, shivers and side or back pain. In babies, a UTI may cause fever with no other signs.
- Pneumonia: cough, fast breathing, chest pain and breathlessness.
- Skin infections: red, warm, spreading skin (cellulitis).
- Travel-related: malaria, dengue and typhoid after visiting tropical regions.
Non-infectious causes
- Inflammatory and autoimmune diseases: rheumatoid arthritis, lupus, inflammatory bowel disease.
- Medicines: some antibiotics, anti-seizure drugs and others can cause drug fever.
- After vaccines: a low-grade fever for a day or two is common.
- Blood clots, cancers (especially lymphoma and leukemia), and heat stroke.
Fever by age: when a reading needs a doctor
The same number means different things at different ages. Young babies have immature immune systems and may have a serious infection with few other signs.
| Age | Temperature | What to do |
|---|---|---|
| Under 3 months | 38 °C (100.4 °F) or higher, measured rectally | Call a doctor or go to the ER now, even if the baby seems well |
| 3 to 6 months | 39 °C (102.2 °F) or higher, or any fever with poor feeding or unusual sleepiness | Call your doctor the same day |
| 6 months to 2 years | Fever lasting more than 24 to 48 hours, or any fever with worrying signs | Call your doctor |
| Children 2 years and over | Fever lasting more than 3 days, or with worrying signs | See a doctor |
| Adults | 39.4 °C (103 °F) or higher that does not come down, or fever for more than 3 days | See a doctor |
| Older adults | Even a modest rise, or confusion without a high reading | Get checked sooner; older adults often run lower fevers with serious infections |
Which thermometer to use
| Method | Best for | Notes |
|---|---|---|
| Rectal | Babies under 3 months (and up to about 3 years) | Most accurate in infants |
| Oral (under the tongue) | Children over 4 to 5 years and adults | Wait 15 minutes after eating or drinking |
| Ear (tympanic) | Over 6 months | Earwax or a poor angle lowers the reading |
| Forehead (temporal artery) | All ages for screening | Quick, but confirm a borderline reading another way |
| Armpit | Screening only | Reads about 0.5 °C (1 °F) lower; least reliable |
Feeling hot without a fever
If you feel hot, flushed or sweaty but a reliable thermometer reads below 38 °C (100.4 °F), you do not have a fever. Common causes include hot flashes, anxiety, an overactive thyroid (hyperthyroidism), medicines and warm surroundings. Our guide to feeling hot without a fever covers these causes and when to get checked.
When to worry about a fever
Fever itself is rarely harmful. How the person looks and behaves matters more than the number. Signs that point to a serious illness:
- unusual drowsiness, confusion, or a child who will not play or respond even after the fever comes down
- a rash that does not fade under pressure
- a stiff neck, severe headache or sensitivity to light
- fast or labored breathing
- signs of dehydration: no urine, dry mouth, no tears
- severe pain anywhere
How doctors find the cause
A doctor will ask how long the fever has lasted, the highest reading, other symptoms, travel, contacts, medicines and vaccinations. They examine the ears, throat, chest, abdomen, skin and neck. Tests are used when the cause is unclear or the person is unwell: urine tests, blood tests (blood count, CRP, cultures), swabs for flu or COVID-19, a chest X-ray, or a lumbar puncture if meningitis is suspected.
What to do for a fever at home
- Drink plenty of fluids to replace water lost through sweating.
- Rest, and wear light clothing. Do not bundle up, even if you feel cold.
- Fever reducers: acetaminophen (paracetamol) or ibuprofen can make you more comfortable. Follow the label dose. Do not give aspirin to children or teenagers. Avoid ibuprofen if you have kidney problems, stomach ulcers or are dehydrated, and do not give it to babies under 6 months.
- Do not sponge with cold water or alcohol. It causes shivering and can be harmful.
- Stay home until you have been fever-free for at least 24 hours without medicine, to avoid spreading infection.
For more on how fever works, measuring temperature correctly and complications, see the fever condition page.
Frequently asked questions
What temperature is considered a fever?
A temperature of 38 °C (100.4 °F) or higher is a fever. In adults, 39.4 °C (103 °F) or higher is a high fever. For babies under 3 months, any temperature of 38 °C (100.4 °F) or more needs medical attention straight away.
What is a high fever in a child?
In children, 39 °C (102.2 °F) or higher is usually called a high fever, but the number matters less than how the child looks. A child who is alert, drinking and playing between doses of medicine is reassuring. Any fever in a baby under 3 months, or a floppy, drowsy or breathless child, needs urgent care.
How long does a fever last?
Most viral fevers last 2 to 3 days and settle within 5 days. Fever lasting more than 3 days in adults or 5 days in children, or a fever that comes back after going away, should be checked by a doctor.
Should I treat a fever or let it run its course?
Fever helps fight infection, so you do not have to bring it down. Treat it if it makes you or your child uncomfortable. Acetaminophen or ibuprofen relieve discomfort. Focus on fluids and watch for warning signs rather than the exact number.
Why do I feel cold when I have a fever?
As a fever starts, the brain raises the body's temperature set point. To reach it, skin blood vessels narrow and muscles shiver, so you feel cold and shivery even though your core is heating up. Once the fever breaks, you sweat and feel hot. Light layers work better than heavy blankets.
Sources
This guide was written from current patient and clinical guidance, including:
- MedlinePlus: Fever
- CDC: Signs and symptoms of flu
- NHS: High temperature (fever) in adults
- American Academy of Pediatrics (HealthyChildren.org): Fever and your baby
- NICE: Fever in under 5s, assessment and initial management
- Mayo Clinic: Fever, first aid