July 03, 2010

Fever: Classes, Causes and Treatment

Fever, also called pyrexia, is abnormal rise in body temperature of a person above the normal level. Fever itself is not an illness. A fever is a symptom of an underlying condition, most often an infection, but may also occur in an inflammation or even tumour.

What is classified as a 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.

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 children. Fevers can occur in children and adults and can be very common. Fevers are often accompanied by sweating.

Control of body temperature

Control of body temperature is regulated by the thermo-regulatory center in the brain; which sets a normal temperature point for the body. Fever results when pyrogens cause an increase of this set point such that the body sees the new set point as its 'normal' temperature.

It can be damaging especially to children below the age of six years, in whom it may easily cause a convulsion - febrile convulsion.

Hyperpyrexia, hyperthermia and hypothermia

Hyperpyrexia (very high fever) is fever of more than 41.5° C (>106° F). It can develop in people with severe infections, but also in those with central nervous system (CNS) bleeding, local trauma or tumour.

Hyperthermia occurs if there is an unchanged normal set point of the thermo-regulatory center in the brain, but an uncontrolled rise in body temperature that exceeds the body’s ability to lose this extra heat. Thus, the heat source is from outside the body, and the body is unable to lose this extra heat fast enough to maintain a healthy temperature. See examples of causes and also treatment below.

Hypothermia is unintentional drop in the body temperature to below 35° C (95° F). See examples of causes and also treatment below.

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 (4-6 PM) and lowest in the early hours of the morning (6 AM).

Your average body temperature, taken with a thermometer in your mouth, is 37°C, but anywhere between 36.5°C and 37.2°C is often considered normal. Armpit temperatures are 0.2°C to 0.3°C lower than this.

How do I take a temperature?

There are different ways of taking your temperature. The quickest and easiest way of taking a temperature is with a thermometer. The analog thermometer (see image below) is no longer favoured in many western countries, but still found in many other countries. There are also electronic or chemical dot thermometers, which are more in vogue in the western countries.

An analog medical/clinical thermometer showing the temperature of 38.7 °C
(image credit: Menchi via Wikimedia Commons)
A thermometer can be placed either under your armpit (e.g. the analog) or in your ear (e.g. the electronic). 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 if you fair-complexioned. This may be difficult to appreciate in a dark-skinned individual.
  • Check for general symptoms of infection, such as aches and pains, tiredness or muscle weakness.
  • Check your urine - 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 in your home.

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: Includes upper respiratory tract infections like the common colds and lower respiratory tract infections like pneumonia. Also see the separate article, Natural Remedies for the Common Cold.
  • Flu (influenza) and Flu-like Illness.
  • Infection of the gut (gastroenteritis).
  • Ear infections.
  • Infection of the tonsils, including tonsillitis.
  • Kidney or urinary tract infections: Including urinary tract infections and pyelonephritis.
  • Common childhood illnesses: for example, chickenpox (varicella), scarlet fever, rubella and whooping cough (pertussis).
If you have recently travelled outside your country and develop a fever, it is important to see a doctor. Infections sometimes caught abroad can also cause a fever. Examples include 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 (see vaccines and vaccinations), or if they overheat because of too much bedding or clothing.

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. This condition is called Deep Vein Thrombosis (DVT) if in the leg, and Pulmonary Embolism (PE) if in the lung.

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. Examples include Rheumatoid Arthritis and Systemic Lupus Erythematosus (SLE).

There are also some types of cancer which can lead to a fever developing. Examples are leukaemia and lymphomas.

Hyperthermia may be due to working or exercising in abnormally hot environments; taking recreational drugs like cocaine; inhalational anaesthesia, thyrotoxicosis; some types of stroke; pheochromacytoma; status epilepticus; and many others.

Hypothermia may occur in a previously healthy person who is exposed to severe cold like in the elderly, the very young, and as an occupational hazard e.g. the military; or when it complicates serious systemic disease, like for example hypothyroidism, hypoglycaemia, hypopituitarism, and sepsis.

How is a fever treated?


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 against infection.

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.


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 in meningitis).

Note: in children, giving paracetamol or ibuprofen does not reduce the risk of a seizure caused by a fever occurring. Also see the separate article, 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).

Hyperthermia and hypothermia

For hyperthermia, physical cooling with sponging, fans, cooling blankets, and even ice baths should be initiated. An urgent visit to the doctor for more specific and technical measures is indicated.

Hypothermic individuals must be rewarmed by covering and insulating them in a warm environment. Hot water bottles may be used. Seek expert medical help urgently.

When should you see a doctor?

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). This could be a sign of serious disease as in measles, HIV, typhoid fever, systemic lupus erythematosus (SLE), syphilis, drug reaction and many others.
  • 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.

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 the family doctor.

1). NICE Guidance (November 2019): Fever in under 5s: assessment and initial management. Available online: https://www.nice.org.uk/guidance/ng143
2). 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. Available online: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=28862659
3). 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. Available online: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=26446754
4). 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. Available online: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=27411542

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