What is a Fever?

What is a Fever?
imagesBy Medical News Today
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
Body temperature is determined by
the body’s thermo regulatory 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
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
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
Measurement Location: Anus or Ear
Average Temperature: 37.6°C
Fever Temperature: 38.0°C
Measurement Location: Mouth
Average Temperature: 36.8°C
Fever Temperature: 37.5°C (99.5°F)
Measurement Location: Armpit
Average Temperature: 36.4°C
Fever Temperature: 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
• Other medical conditions such as
skin inflammation, arthritis,
hyperthyroidism, some cancers,
lupus, inflammatory bowel disease,
metabolic disorder, gout, and
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
Patients with fever should be sure
to remain hydrated. Since fever
often causes discomfort and
increases heart rate and
metabolism, many people tak
antipyretics such as ibuprofen org
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
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.