Average body temperature is about 98.6°F (37°C), and temperatures above 100.4°F (38°C) are considered to be a fever (febrile).
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 and supporting information is in the main article.
- common symptoms include chills and shaking
- bacterial or viral infections are common causes
- not all fevers require treatment
Symptoms of fever
A fever may be a cause of concern, but it is rarely a reason for panic. Exceptions include if it occurs in a very young child, if it continues for more than 3 days, or if the temperature is so high that it approaches heatstroke risk.
General signs and symptoms associated with a fever include:
- elevated temperature
- shivering, chill, shaking
- intermittent or excessive sweating
- skin flushing
- feeling dizzy or faint
Due to the variable nature of body temperature measurements, doctors may look for other signs of sickness that can accompany fever. These include:
- low appetite
- increased pain sensitivity
- inability to concentrate
If a child under 4 months of age has a rectal temperature of over 100.4°F they should be seen by a doctor immediately.
Other warning signs in young children that require immediate attention include:
- if they look very ill
- if they are very drowsy or fussy
- if they already have a weak immune system
- if they have a seizure
- if they have a sore throat, rash, stiff neck, earache, or headache
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, and the body perceives itself as suffering from hypothermia (colder than it should be).
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, increased muscle tone (stiffness), and shivering.
Body temperature is controlled by a part of the brain called the hypothalamus. The hypothalamus sends signals to the body telling it to warm up or cool down. When the body needs to warm up after its thermal set-point is raised (the fever), the hypothalamus instructs it to increase muscle tone, shiver, secrete hormones such as epinephrine, and constrict blood vessels.
How is body temperature measured?
Body temperature is measured using a thermometer, which is placed in the mouth, ear, anus, or arm. However, the readings may not all be the same.
A person's body temperature will also change 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.
Body temperature may also depend on the time of day, with it 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, ear||99.6°F (37.6°C)||100.4°F (38.0°C)|
|Mouth||98.2°F (36.8°C)||99.5°F (37.5°C)|
|Armpit||97.6°F (36.4°C)||99.0°F (37.2°C)|
Causes of fever
There are several conditions, illnesses, and medicines that can cause fever. These include:
- Infections and infectious diseases, such as influenza, common cold, HIV, malaria, infectious mononucleosis, and gastroenteritis. Infections are the most common cause of fever.
- Medicines, such as antibiotics, narcotics, barbiturates, and antihistamines. These cause "drug fevers" due to adverse reactions, withdrawal, or by the drug's design.
- Illegal drugs including amphetamines and cocaine.
- Trauma or injury, such as a heart attack, stroke, heatstroke, heat exhaustion, or burns.
- Damage to tissue, such as from hemolysis (breaking open of red blood cells to release hemoglobin), surgery, heart attack, crush syndrome, and hemorrhage.
- Other medical conditions, such as skin inflammation, arthritis, hyperthyroidism, some cancers, lupus, inflammatory bowel disease, blood clots, metabolic disorder, gout, and embolisms.
Fevers that exist for days or weeks with no explanation are called fevers of undetermined origin (FUO).
Not all fevers require treatment. In fact, fever is an important indicator that there is something wrong with the body, and it is often used to gauge the success of medical treatments.
Fevers may also be useful because they increase the amount of antiviral and anticancer interferon in the blood, making it difficult for bacteria and viruses to replicate. In other words, a fever can help fight some diseases and, therefore, should not be treated in some cases.
Patients with fever should remain hydrated. Since fever often causes discomfort and increases the heart rate and metabolism, many people take antipyretics, such as ibuprofen or acetaminophen. Aspirin is used as a treatment for adults but not in children due to the risk of Reye's syndrome.
In cases where fever escalates so high that tissue damage is likely, the fever must be brought under control.
Infants, aged from birth to 3 months, are considered febrile with a rectal temperature of 100.4°F (38°C), and a doctor should be notified immediately.
Children, classified as aged 3 months to 18 years, should rest and stay hydrated if they have a temperature lower than 102°F (38.9°C), taken orally. Higher fevers may require acetaminophen or ibuprofen.
It is recommended in the Practice guideline for the management of infants and children 0 to 36 months of age with fever without source that a doctor should be notified if the child does not respond to medicine or if they seem unusually lethargic, irritable, or uncomfortable.
Adults should also rest and remain hydrated with fever, and may take acetaminophen, ibuprofen, or aspirin to reduce their raised temperature. Adults should consider notifying a doctor if fever is accompanied by concerning signs and symptoms, such as stiff neck, severe headache, altered mental status, joint swelling, burning with urination, chest pain, productive cough, abdominal pain or if the fever is consistently high for more than 3 days.
Although a fever is easy to measure, 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.