A high temperature is called a fever (or sometimes a high fever). Fever itself is not an illness. A fever is usually a symptom of an underlying condition, usually an infection.
What is a fever?
Body temperature fluctuates slightly throughout the day. Normal temperature varies depending on the person, the body site where temperature is measured, and the time of day. A temperature above the normal range is known as a fever.
What temperature is a fever?
It is usually agreed that a person with a temperature of 38°C or higher has a fever. However, a temperature over 37.5°C can be considered to be a mild fever. Fevers occur in both children and adults and are very common, particularly in children. Fevers are often accompanied by sweating.
What is a normal temperature?
The normal body temperature changes throughout the day. These changes can be caused by exercise, eating, sleeping and even the time of day. The temperature is usually highest in the early evening and lowest in the early hours of the morning.
The average body temperature, taken with a digital thermometer in your mouth, is 37°C (98.6°F), but anywhere between 36.5°C (97.7°F) and 37.2°C (98.96°F) is considered normal. Armpit temperatures tend to be 0.2°C to 0.3°C lower than this.
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What causes a fever?
High temperature (fever) is caused by the release of certain chemicals by the immune system, usually as a result of infection or inflammation.
Most fevers are caused by infections or other illnesses. Viral infections are the commonest causes of a fever. The high body temperature makes it more difficult for the germs (bacteria or viruses) which cause infections to survive.
Fever caused by infections
Common conditions that can cause fevers include:
- Respiratory tract infections and colds: see the separate leaflet called Common Cold (Upper Respiratory Tract Infections).
- Flu (influenza): see the separate leaflet called Influenza and Flu-like Illness.
- Infection of the gut (gastroenteritis): see the separate leaflets called Gastroenteritis in Children and Gastroenteritis.
- Ear infections: see the separate leaflet called Ear Infection (Otitis Media).
- Infection of the tonsils: see the separate leaflet called Tonsillitis.
- Kidney or urinary tract infections: see the separate leaflets called Urine Infection in Children, Cystitis in Women, Urine Infection in Pregnancy, Urine Infection in Men, Kidney Infection (Pyelonephritis) and What do kidneys do?
- Common childhood illnesses: for example, chickenpox (varicella), scarlet fever, rubella and whooping cough (pertussis). See the separate leaflets called Chickenpox in Children, Scarlet Fever, and Whooping Cough.
If developing a fever after having been abroad, it is important to seek medical advice. Certain infections that can be caught abroad (such as Malaria, typhoid or yellow fever) can cause a fever.
A child's temperature can also be raised when their teeth start to develop (their teething period), following some vaccinations, or if overheating because of too much bedding or clothing. See the separate leaflets called Teething, Immunisation and Sudden Infant Death (Cot Death).
Fever caused by other conditions
Fever can be caused by conditions other than infections but this is less common. For example, blood clots in the leg or lung can occasionally lead to a fever developing. See the separate leaflets called Deep Vein Thrombosis and Pulmonary Embolism.
There are also some types of cancer which can lead to a fever developing. See the separate leaflet called Cancer.
How common is a fever?
A high temperature (fever) is very common, particularly in children. Around 3 out of 10 young children have a fever every year. Older adults are less likely to develop a high temperature when they get an infection; however, older adults who do develop a fever with an infection have been shown to be more likely to recover quickly.
How do I take my temperature?
There are different ways of taking a temperature with a thermometer. A thermometer can be placed either under your armpit or in your ear. The forehead thermometers are no longer recommended as they are less accurate. Rectal thermometers are no longer usually used in the UK.
As a parent it can feel worrying if a child has a high temperature (fever). However, having a fever is very common and usually settles by itself without treatment.
How do you know if you have a fever without a thermometer?
Using a thermometer is the best way to tell if there is a fever. However the following will give a rough idea:
- The forehead feels hot when felt with the back of the hand. This is much more sensitive to temperature than the palm.
- Cheeks may be flushed.
- Feeling hotter than others around.
How to reduce a fever
Note: Fever usually responds well to medication such as paracetamol and ibuprofen. However, these medicines do not "cure" a fever and, as soon as the medication wears off, the temperature will increase again. This is normal and, if wanting to reduce the fever, then regular medication will need to be given.
Fever in adults
A high temperature (fever) is usually associated with physical discomfort and most people feel better when a fever is treated. However, depending on age, physical condition and the underlying cause of the fever, it may be best not to treat the fever. Many experts believe that fever helps the body fight off infections naturally.. There is some evidence that fever reduces the severity of infections and helps faster recovery. However, paracetamol and ibuprofen will both reduce the fever if needed. It is important to drink plenty of fluids to avoid becoming dehydrated from sweating.
If there is an obvious cause for the fever, for example a flu-like illness with aches and pains and headaches, then treating the symptoms with paracetamol or ibuprofen will also help the underlying fever. If there is a bacterial infection, for example a urine infection, then antibiotics will be needed to treat the infection but these will not reduce the fever itself.
Tests may sometimes be needed if there is no obvious cause for the fever.
Testing may include:
Fever in children
In children, paracetamol and ibuprofen are only recommended if the child is distressed with the fever.
Warm (tepid) sponging is no longer recommended for children with a fever. Drinking plenty of fluids is important during a fever. Keeping fluid levels up is important to reduce the risk of dehydration. It is also important to make sure they are not wearing too many clothes - one layer is fine.
Note: in children, giving paracetamol or ibuprofen does not reduce the risk of a seizure caused by a fever occurring. See the separate leaflet called Febrile Seizure (Febrile Convulsion).
When should you see a doctor about a fever?
Note: Fever itself is not harmful. Many people think they need to seek medical advice when a temperature reaches, for example, 39 degrees in a toddler. If there is an obvious cause for the temperature (for example, a cold with a runny nose) then medical advice does not need to be sought just because of the height of the temperature. This advice is different for babies under 6 months of age.
You should contact a doctor if there is a high temperature (fever) and:
- A child is under 3 months of age.
- A child is 3-6 months of age with a fever of 39 degrees or more.
- A child has a fit.
- There is a rash that does not disappear when pressing a glass on it (a non-blanching rash), This is a potential medical emergency that requires attendance at an emergency department.
- The person with the fever is becoming more unwell, for example with shortness of breath or chest pain.
- A fever lasts longer than five days.
- There is no obvious cause of the fever - for example, no cough or cold symptoms.
- There may be a bacterial infection causing of the fever - for example, a urinary tract infection or a red hot swollen joint.
Further reading and references
Fever in under 5s: assessment and initial management; NICE Guidance (last updated November 2021)
Barbi E, Marzuillo P, Neri E, et al; Fever in Children: Pearls and Pitfalls. Children (Basel). 2017 Sep 14(9). pii: children4090081. doi: 10.3390/children4090081.
Walter EJ, Hanna-Jumma S, Carraretto M, et al; The pathophysiological basis and consequences of fever. Crit Care. 2016 Jul 1420(1):200. doi: 10.1186/s13054-016-1375-5.
Fever and High Temperatures; Alder Hey Hospital
Fever in the Elderly; Norman