What is fever?

Most people know about fever is but may have trouble explaining what it is.  Because fever is a common medical condition, it is worthwhile to understand what it is, why it happens and what it means.

What is Fever?

Fever is defined as an elevation of body temperature above the normal range of 36.5–37.5 °C (98–100 °F). It is triggered by a signal from the temperature regulatory center in the brain in response to a message from the body that there is a threat to the body’s well-being. Raising temperature is the body’s way of fighting back. Many bacteria and viruses function best at regular body temperature and the defensive action of the body in raising the temperature may make them less efficient, or it may decrease levels of certain nutrients or minerals in the blood that they need to function. Some tumors can also be damaged by increased body temperature, and artificial, medically produced elevation of body temperature, called hyperthermia, is sometimes used to treat certain cancers.

The signal to the brain to raise temperature is most often caused by a substance released by or in response to an infection by a bacteria or a virus. In response to this threat, the brain sets a new, higher temperature as its goal and it sends signals to the muscles to cause activity such as shivering. Shivering produces heat and raises the body’s temperature. Even though the temperature is rising, the sick person may feel cold and even experience chills or shivers, because the brain perceives the current body temperature as too low, and because the desired temperature has not yet been reached. Once the new temperature goal is reached, the chills go away and there is a sensation of warmth. At that point, the brain may decide that the new temperature is too high and directs the body to sweat in order to lose some heat. So when the temperature is rising, the sick person feels himself to be cold, and when it is falling, he or she feels hot.

However, fever can also be due to inflammation or auto-immune illness, such as lupus, in which the immune system is dysregulated, or in response to substances secreted by tumors. In such cases, fever becomes a part of the illness process. Even when fever is a part of fighting illness, it can go too high and cause damage.

 How is fever measured?

Different parts of the body have slightly different temperatures. In the United States, temperature is usually measured with a thermometer in the mouth or rectally; in some Eastern European countries temperatures are commonly measured in the armpit and normal temperatures there are slightly lower, because armpits are farther from the core of the body. The temperature in the mouth is also slightly lower than in the core. The commonly accepted average core body temperature (taken rectally) is 37.0 °C (98.6 °F). The typical oral (under the tongue) measurement is 36.8C (98.2F). Since the year 2000, small ear thermometers have become available and they take the core temperature. They work by detecting the infrared heat emission from the ear membrane. They provide an immediate readout and are popular with children and pediatricians.  In hospitals, daily measurements of  body temperature is one of the vital signs, which also include blood pressure, pulse( how many heart beats per minute),  and respiratory rate (how many breaths per minute).

When should fever be treated?

Because fever is a symptom and not a disease, it is imperative to understand the cause of fever before it is treated. Remember that fever is helping to fight  infection, so use fever medications only if the fever is high or if it is in the  moderate range and the adult or the child with fever is uncomfortable. In general, you should call your pediatrician if your infant under three months of age has a rectal temperature above 100.4 F, if your infant aged 3-6 months has a temperature above 101 F, or if an infant above 6 months has a temperature above 103 F. For older children, if your older child is alert, active and playful, is not having difficulty breathing, and is eating and sleeping well, or if the temperature comes down quickly with home treatments (and he is feeling well), then you don’t necessarily need to call your doctor immediately.

Most fevers in healthy adults are transitory and harmless but they can be uncomfortable. Once the diagnosis of what is causing the fever is established, Tylenol (acetominophen) or other fever lowering medications can be used for comfort.  Aspirin should be avoided in children to treat the fever of viral infections because it can rarely cause Reye’s syndrome, a serious complication.

Keep in mind that infection and fever responses in children and the elderly may vary. Some older patients have a decreased or no-fever response to infection, and children usually develop higher temperatures than adults with minor infections.

Fever should also be treated when it exacerbates an underlying condition. For example, if the patient has a heart condition, a fever may increase his or her heart rate, stress the heart and increase the risk of suffering a heart related complication. Patients with fever, especially the elderly, loose more moisture in the form of sweat and should increase their fluid intake.

A medical evaluation is warranted for adults if the temperature:

* rises to 104 deg F (40 deg C or above)

* stays at or above 101 deg F (38.3 deg C) for more than 3 days

* stays at or above 100.5 deg F (38.1 deg C) for 3 weeks or more, even if there are no other symptoms, or

* is accompanied by other symptoms or discomfort.

Fever can produce fatigue and dehydration. High fevers can produce convulsions, especially among young children, but they are rarely dangerous. Brain damage from a fever generally will not occur unless the fever is over 107.6 °F (42 °C). It is rare for a fever from an infection to go over 105 °F unless the child is overdressed or trapped in a hot place. Temperature that approaches 107 degrees F is considered a medical emergency.

A fever is usually accompanied by weakness, depression, loss of appetite, sleepiness, generalized discomfort and inability to concentrate. This is why the decision to bring down a fever is best individualized in consultation with a physician. Allowing a mild fever to remain somewhat shortens the duration of an illness but it makes it more uncomfortable. It is important to remember, however, that the goal of treating fever is solely to prevent it from rising too high or to provide comfort.

In the past, before medical science sharply separated diseases and the symptoms they produced, many conditions were called “fevers”. Even now one encounters the name of fever as a name of certain illness, for example, Yellow Fever, Rheumatic Fever and the like. However, medical science now sees fever as indicating that there is an illness, not as an illness itself. Consequently, it is imperative to establish a diagnosis before tackling fever.


Fever is a manifestation of an illness and not a disease in itself. Most often it is a friend of the sick individual and helps speed recovery. However, there are times when it is a hindrance to recovery or can cause complications. A competent medical professional should be consulted when the fever is high, is present in an infant, pregnant woman or the elderly, or when it does not go away.


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