What is a Fever? Fever Symptoms and Causes

What is a fever?

A fever, or pyrexia, is a rise in internal body temperature to levels that are considered to be above normal. Average body temperature is about 98.6°F or 37°C, and temperatures above 100.4°F or 38°C are generally considered to be febrile.

Body 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 this 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, common fever symptoms exist such as feelings of cold, increased heart rate, increased muscle tone (stiffness), and shivering.

A fever may be a cause of concern, but it is rarely a reason for panic unless it is occurring in very young children, sustained for more than three days, or so high so to approach heatstroke risk.

How does the body regulate temperature?

Body temperature is regulated by a part of the brain called the hypothalamus. The hypothalamus sends signals to the rest of 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 the body 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. Readings are usually taken in the mouth, ear, anus, or arm, but the readings may not all be the same. Your body temperature will also change after eating, during periods of high activity levels, with different clothing, after smoking, in warm or cold rooms, at various points in the menstrual cycle, and depending on the time of day. Your temperature in the morning is often lower 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 that usually is considered febrile or feverish for an adult.

Measurement Location	Average Temperature	Fever Temperature
Anus, vagina, ear	37.6°C (99.6°F)	38.0°C (100.4°F)
Mouth	36.8°C (98.2°F)	37.5°C (99.5°F)
Armpit	36.4°C (97.6°F)	37.2°C (99.0°F)

Because of the variable nature of body temperature readings, physicians often look for other signs of sickness that would accompany fever such as lethargy, depression, low appetite, sleepiness, increased pain sensitivity, and a decreased ability to concentrate.

What causes 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.


• Trauma or injury such as a heart attack, stroke, heatstroke, heat exhaustion, or burns.


• Damage to tissue 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, metabolic disorder, gout, and embolisms.

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

How is fever treated?

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 in order 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.

Patients with fever should be sure to remain hydrated. Since fever often causes discomfort and increases heart rate and metabolism, many people take antipyretics such as ibuprofen or acetaminophen (paracetamol). 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.

The general guidelines for diagnosing and treating fever differ depending on the age of the patient. Infants from birth to three months are considered febrile with a rectal temperature of 100.4° F or 38° C, and a doctor should be notified immediately. Children from three months to 18 years should rest and remain hydrated if they have a fever lower than 102° F or 38.9° C taken orally. Higher fevers may require acetaminophen or ibuprofen. A doctor should be notified if the child does not respond to medicine or if she seems unusually lethargic, irritable, or uncomfortable. Adults should also rest and remain hydrated with fever and may take acetaminophen, ibuprofen, or aspirin to reduce fever. Adults may consider notifying a doctor if fever is accompanied by stiff neck, severe headache, or if the fever is consistently high for more than three days.