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, most often an infection.
What isa fever?
Your body temperature fluctuates slightly throughout the day. A high temperature is also known as a fever. Generally, a fever is a rise of body temperature above the normal daily variation. Normal temperature varies depending on the person, the body site where temperature is measured, and the time of day.
What temperature is a fever?
The actual temperature causing a fever varies between different people and depends on what their usual temperature is. 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 often be considered to be a fever in many children. Fevers can occur in children and adults and can be very common. Fevers are often accompanied by sweating.
What is a normal temperature?
Your normal body temperature changes throughout the day. These changes can be caused by exercise, eating, sleeping and even the time of day. Your temperature is usually highest in the early evening and lowest in the early hours of the morning.
Your average body temperature, taken with a 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 often considered normal. Armpit temperatures are 0.2°C to 0.3°C lower than this.
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How do I take my temperature?
There are different ways of taking your temperature. The quickest and easiest way of taking a temperature is with a thermometer. This can be by an electronic or chemical dot thermometer. A thermometer can be placed either under your armpit or in your ear. The forehead thermometers are no longer recommended.
As a parent it can be extremely worrying if your child has a high temperature (fever). However, having a fever is very common and often clears up 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 you have a fever. However, if you don't have one, the following will give you a rough idea:
- Feel your forehead with the back of your hand. This is much more sensitive to temperature than the palm.
- Look in the mirror. If you have a fever, your cheeks may be flushed.
- Check for general symptoms of infection, such as aches and pains, tiredness or muscle weakness.
- Check your wee - if it's darker than usual you may be getting dehydrated from excess sweating.
- Ask anyone around if they feel hot - obvious, really but if they do, it could just mean you need to turn down the heating.
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 your age, physical condition and the underlying cause of your fever, you may or may not require medical treatment for the fever alone. Many experts believe that fever is a natural bodily defense for fighting infections.
Your doctor will examine you and try to determine the underlying cause for your fever. If it is due to infection with a germ (a bacterial infection) - for example, a urine infection - you may be given antibiotics. You may need to have some tests, especially if a cause other than an infection is likely. Testing may include:
- Blood tests.
- Urine tests.
- X-rays to determine the cause, especially where a cause other than an infection is a possibility.
Paracetamol or ibuprofen may help to reduce the fever.
Fever in children
In children, paracetamol and ibuprofen are usually only recommended if the child is distressed with the fever. Remember that just because a fever responds to medicine, this does not mean that you should stop looking out for signs of serious illness (such as a non-blanching rash).
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).
Warm (tepid) sponging is no longer recommended for children with a fever. Drinking plenty of fluids is important when you (or your child) have a fever. Keeping fluid levels up is important to reduce the risk of lack of fluid in the body (dehydration).
When should you see a doctor about a fever?
You should contact a doctor if you or your child have a high temperature (fever) and:
- Your child has a fit.
- You or your child develop a rash that does not disappear when you press a glass on it (a non-blanching rash).
- You or your child are becoming more unwell.
- The fever lasts longer than five days.
You should contact a doctor if you have a fever and have worsening symptoms - for example, a rash, stiff neck, shortness of breath or chest pains.
What causes fever?
High temperature (fever) is caused by the release of certain chemicals by your immune system, usually as a result of infection or inflammation. Fever is an important sign that a person is unwell and a cause should usually be found.
Most fevers are caused by infections or other illnesses. Viral infections are very common 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, Rubella (German Measles) and Whooping Cough.
If you have been abroad and develop a fever, it is important to see a doctor. Infections sometimes caught abroad can also cause a fever. See the separate leaflets called Malaria, Tuberculosis, Hepatitis A, Hepatitis B and Lyme Disease.
Your child's temperature can also be raised when their teeth start to develop (their teething period), following some vaccinations, or if they overheat 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
Less commonly, fever can be caused by conditions other than infections. For example, blood clots in the leg or lung can sometimes lead to a fever developing. See the separate leaflets called Deep Vein Thrombosis and Pulmonary Embolism.
There are some conditions which cause swelling (inflammation) in the body. If you have one of these conditions, a common symptom you may experience is fever. See the separate leaflets called Rheumatoid Arthritis and Systemic Lupus Erythematosus.
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) can be really common. They are more common in children. Around 3 out of 10 young children have a fever every year and a fever is one of the most common reasons for a child to be seen by their GP.
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.
de Bont EG, Loonen N, Hendrix DA, et al; Childhood fever: a qualitative study on parents' expectations and experiences during general practice out-of-hours care consultations. BMC Fam Pract. 2015 Oct 716:131. doi: 10.1186/s12875-015-0348-0.
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.