Fever, or pyrexia, is when the internal body temperature rises to levels that are considered above normal.

It normally indicates the present of another condition, usually an infection.

Average body temperature is about 98.6 °Fahrenheit (37 °Celsius), and temperatures above 100.4 °F (38 °C) are considered to be a fever. Three common sites for checking temperature are the mouth, under the arm, and the rectum.

Although unpleasant, a fever on its own is not generally considered dangerous. However, in young children, older adults, or individuals with other health concerns, a fever should be checked out by a doctor.

Fast facts on fever

Here are some key points about fever. More detail is in the main article.

  • Common symptoms of a fever include chills and shaking
  • Bacterial or viral infections are common causes.
  • A fever can be part of the body’s way to fight infection.
  • Aspirin and acetaminophen can help reduce fever, but aspirin is not suitable for children.

A fever is not usually a problem in itself, but a symptom of another condition. It indicates that there is something wrong with some part of the body.

It can happen when something goes wrong with one of a wide range of functions.

As one researcher explains, “The febrile response is orchestrated by the central nervous system through endocrine, neurological, immunological, and behavioral mechanisms.”

A fever can help the body fight infection, because it increases the amount of antiviral and anticancer interferon in the blood. This makes it difficult for bacteria and viruses to replicate.

Body temperature can also help measure the success of medical treatments.

Conditions that cause a fever

Infections are the most common cause of fever, but various conditions, illnesses, and medicines can raise the body temperature.

These include:

Antibiotics, narcotics, barbiturates, and antihistamines can cause “drug fevers” due to adverse reactions, withdrawal, or because of the design of the drug.

How does the body regulate temperature?

A person’s temperature is determined by the body’s thermoregulatory set-point.

The body increases this set-point in response to threats, such as bacterial or viral infections. When the set-point rises, a fever occurs. At this point, the body believes it has hypothermia. It perceives that it is too cold.

As the body works towards meeting the new temperature set-point, symptoms commonly associated with a fever emerge, such as feelings of being cold, increased heart rate, stiffness, and shivering.

Body temperature is controlled by a part of the brain called the hypothalamus, and a network of systems also play a key role. These form a circuit that regulates the temperature. The hypothalamus sends signals to the body telling it to warm up or cool down.

Fevers can be classified in different ways.

One way is the length of time. A fever can be:

  • acute, lasting less than 7 days, as in a viral upper respiratory tract infection
  • sub-acute, lasting up to 14 days, as, for example, in typhoid
  • chronic or persistent, lasting over 14 days, as in tuberculosis, HIV, and cancers

They can also be classed according to severity:

  • low grade
  • moderate
  • high
  • hyperpyrexia

The height of the temperature may help indicate what type of problem is causing it.

Fevers can also be:

  • sustained or continuous, where it does not fluctuate more than 1.5 °F (1 °C) over 24 hours, but is never normal in this time
  • intermittent, when the fever occurs for several hours in the day, but not all the time
  • remittent, when it fluctuates by more than 2 °C but does not become normal

Typhoid may underlie a sustained fever, tuberculosis tends to cause an intermittent fever, and infective endocarditis may trigger a remittent fever.

Fevers that exist for days or weeks with no explanation are called fevers of undetermined origin (FUO).

A fever is a symptom, but it can occur with its own symptoms and with other symptoms.

A person who has a fever may also have:

  • a high temperature
  • shivering, chills, and shaking
  • intermittent or excessive sweating
  • skin flushing
  • palpitations
  • feeling weak, dizzy, or faint

Body temperature measurements vary, so doctors may look for other signs of sickness that can accompany fever.

These include:

  • lethargy
  • depression
  • low appetite
  • sleepiness
  • increased pain sensitivity
  • inability to concentrate

Young children with a fever should see a doctor, but older children and adults do not always need to do this. They should rest and remain hydrated. Adults may take acetaminophen, ibuprofen, or aspirin to reduce the temperature. Children should not take aspirin.

A person with a fever is at risk of dehydration, so they should drink plenty of fluid.

It may help to:

  • remove some clothes
  • cool the room, for example, using a fan
  • put a cool damp cloth on the forehead
  • take a cool bath

If a child aged 0 to 36 months has a fever, they should see a doctor.

If an adult has a fever accompanied by other symptoms, they should see a doctor.

Symptoms may include:

  • a stiff neck
  • severe headache
  • altered mental status
  • joint swelling
  • burning with urination
  • chest pain
  • productive cough
  • abdominal pain
  • a fever that is consistently high for more than 3 days

In cases where fever escalates so high that tissue damage is likely, the fever must be brought under control.

Treatment will depend on the cause of the fever.

Normal body temperature is around 98.6–100.4 °F or 37-38 °C.

The following temperatures indicate different levels of fever:

Temperature°F°C
Mid- to low-grade fever100.5-102.238.1-39
Moderate-grade fever102.2-104.039.1-40
High-grade fever104.1-106.040.1-41.1
Hyperpyrexiaover 106.0over 41.1

How is body temperature measured?

Body temperature is measured by placing a thermometer in the mouth, ear, anus, or arm. The readings may not all be the same.

Body temperature also varies after eating, during periods of high activity levels, with different clothing, after smoking, in warm or cold rooms, and at various points of a woman’s menstrual cycle.

It may also depend on the time of day. It is often lower in the morning than during the day or in the evening.

With the above variations in mind, the table below summarizes the average daytime temperatures for each measurement location and the temperature usually considered febrile or feverish for an adult.

Measurement area Average Temp Fever Temp
Anus, vagina, ear99.6°F (37.6°C)100.4°F (38.0°C)
Mouth98.2°F (36.8°C)99.5°F (37.5°C)
Armpit97.6°F (36.4°C)99.0°F (37.2°C)

If a child under 4 months of age has a rectal temperature of over 100.4° F or 38° C, they should be seen by a doctor immediately.

Other warning signs in young children that require immediate attention include:

  • look very ill
  • being drowsy or fussy
  • already having a weakened immune system
  • having a seizure
  • having a sore throat, rash, stiff neck, earache, or headache

If a child aged 0 to 36 months has a fever, they should see a doctor if the cause has not already been clarified or if they have other symptoms, such as lethargy or a rash.

Aspirin is not suitable for children due to the risk of Reye’s syndrome.

A fever is easy to measure, but it can be difficult to determine its cause. A doctor will ask about other symptoms, medications, recent travels, and other infection risks in order to diagnose what the cause might be. Any underlying cause that needs treatment can then be dealt with.