The complete guide for fever: Definition, causes, symptoms and other facts

Fever, also known as pyrexia, hyperthermia or simply raised temperature, is the commonest health-related issue or a symptom. From a minor self-limiting flu to a fatal cancer, fever could be a symptom for a lot of health conditions. And for this reason alone, even if easily taken care with an over-the-counter drug, fever still sets fear in the people’s heart.

How do we define fever?

Fever can be defined as a temporary rise in the body temperature, usually a result of underlying illness. Commonly, fever is the earliest manifestation of something abnormal going on with the body.

To categorize the rise in temperature as fever different experts have their own criteria. Most commonly the temperature levels to define fever are:

In children

  • Oral temperature greater than or equal to 99.5 °F/37.5 °C
  • Rectal temperature greater than or equal to 100.4°F/38°C
  • Axillary (armpit) temperature greater than or equal to 99°F/37.2°C

In adults the temperature might vary with time of the day, thus the range depending upon the time is greater than or equal to 99°F/37.2°C to 99.5°F/37.5°C

Although an adult is able to tolerate fever well up to 102-103°F, in children even slight rise in temperature mandates medical attention. Hence, it’s appropriate for everyone to know the basics of fever including when to visit the doctor or emergency hospital.

In what conditions can temperature rise but not considered as fever

  1. Diurnal variations: The body temperature changes throughout the day, mostly maximum during the evening.
  2. Hormonal variation in women: In women the hormonal changes that occur throughout a menstrual cycle can bring significant change the body temperature. The temperature can rise by one degree during the second half of the cycle. Usually around the time of ovulation (commonly 14th day) the temperature is maximum.
  3. Temperature can also rise under many other circumstances like during emotional outburst, rigorous activities, on medications, in hot and humid environment, on wearing hefty clothing, and while eating.

Some facts you must know about fever

  1. Fever is not our enemy. In fact, it is our body’s soldier against infections and other conditions. Most pathogens require same temperature as our normal temperature. Hence, during infections, as part of defense system, body temperature is raised to make conditions unfavorable for the microbes. If someone has fever then it means than the body is fighting against an infection or another condition and underlying problem must be treated.
  2. In some cases, the fever continues for days to week and will remain undiagnosed even after intense diagnostic methods. Such a fever is labeled as pyrexia of unknown origin (PUO).
  3. High temperatures may lead to brain damage especially in children. However, it occurs usually requires temperature as high as 107°F. Good thing is even if untreated temperature wouldn’t go that high as long as the child isn’t overdressed in warm clothes or surrounding is too hot.
  4. Higher temperature increases the risk of afebrile seizures. Nevertheless, an episode of afebrile seizures doesn’t mean that the child has or will develop epilepsy.

What are the Causes of Fever?

The mechanism of fever development includes the setting of our body’s thermoregulation controlled by hypothalamus to higher level. It makes one feel colder, and thus both voluntarily and involuntarily we try to keep ourselves warm. That is how the fever develops. Fever is actually a protective method employed by our body to fight against problems like infections.

Let us take a look at some of the conditions leading to fever.

  1. Among all the causes of fever infection holds the first place. Nearly all kinds of infections cause fever. Some of these infections where fever is prominent are as follows:
    • Respiratory infections like common cold, flu, pneumonia, bronchitis, mononucleosis, sinusitis, sore throat, tuberculosis
    • Bone infection like osteomyelitis
    • Skin infections like cellulitis
    • Abdominal infections like viral gastroenteritis, bacterial gastroenteritis and appendicitis
    • Meningitis
    • Infections of the urinary tract
    • Post immunization in children (low-grade fever generally)
  2. Many a time fever might be the first presentation of a cancer especially in leukemia, non-Hodgkin lymphoma and Hodgkin lymphoma.
  3. Children may suffer from low-grade fever during teething, though the temperature seldom reaches 100°F.
  4. Various autoimmune disorders and inflammatory disorders may represent as fever too. Some of them are as follows:
    • Connective tissues disorder like systemic lupus erythematosus or rheumatoid arthritis
    • Crohn’s disease or Ulcerative colitis
    • Periarteritis nodosa
    • Vasculitis
  5. Other causes of fever are:
    • Thrombophlebitis
    • Medications like antihistamines, antibiotics, antihypertensive or antiepileptic drugs
    • Extreme sunburn or heat exhaustion


Many a times a proper cause of fever cannot be identified even after extensive diagnostic methods. When the cause is not identified for a fever (over 101°F or more lasting for 3 weeks or more) even after extensive evaluation the it is termed as pyrexia or fever of unknown origin (PUO or FUO).

What are the other signs and symptoms associated with fever?

Well, there is no doubt there that the defining criteria for fever is the abnormal rise in body temperature. You must always remember that what might be normal for one person, might be abnormal for another.

When you’re suffering from fever you must also look out for other signs and symptoms, which helps in understanding the underlying cause of fever:

  1. Chills and rigors: Feeling extremely cold accompanied by shivering. In some condition rigors may occur in absence of chills or vice versa.
  2. Abnormal sweating
  3. Headache
  4. Loss of appetite
  5. Muscular aches
  6. Generalized weakness
  7. Dehydration

In moderate to high grade fever (more than 103°F) fever may be accompanied by symptoms related to nervous system. For example,

  1. Confusion
  2. Hallucinations
  3. Irritation
  4. Febrile seizures

How to measure temperature

There are various types of thermometer available to aid you in taking the temperature like rectal, oral, tympanic (for the ears), and temporal artery (for the forehead) thermometers. Common types of thermometers are regular mercury thermometers or digital thermometers.

[Buy a Digital thermometer here]

Common methods of taking temperature are oral or axillary. Axillary temperature might not be the most accurate method of measuring body temperature; it’s certainly easiest and most convenient. To take axillary temperature you must position the thermometer in the armpit, and cross the arms. Wait for 4-5 minutes before checking the temperature. Make sure when you report the temperature to a doctor you mention both the accurate temperature and the location of measuring temperature.

In infant rectal temperature might be used to record the temperature.

  • Apply a little petroleum jelly to the thermometer bulb.
  • Position the baby with abdomen facing down
  • Be careful while inserting the thermometer bulb about half to one inch into the rectum of the baby
  • Keep holding the thermometer and the baby for as long as it’s inside. If the baby wriggles the thermometer might slip deeper and cause damage.

In the present corona affected era another kind of thermometers getting popularity is Infrared forehead thermometer or thermal gun which can detect temperature by flashing the meter before forehead. It is particularly advantageous as neither the thermometer nor the person measuring the temperature needs to touch the patient (or suspect), thus preventing transmission of Corona Virus (or any other contagious infection). However, the downside of this kind of thermometers is accuracy.

[Buy infrared thermometer here]

Complications of fever

Various complications associated with fever are:

  • Hallucinations
  • Severe dehydration
  • Febrile seizures i.e. seizures induced by high fever in children of 6 months to 5 years of age.

Got a fever? When should you see a doctor?

When to see a doctor for fever

Although fever is the body’s protective mechanism, often self-limiting or treatable at home, sometimes it warrants an immediate medical care. It is important know the signs and symptoms that would sent out an emergency call.

In Infants

  1. If the baby is younger than 3 months and the rectal temperature reaches 38°C (100.4°F) or more.
  2. If the baby is aged between 3-6 months and the rectal temperature is 38.9°C (102°F) or more and the baby seems lethargic, irritated or uncomfortable.
  3. If the baby is aged between 6-24 months and the rectal temperature is 38.9°C (102°F) or more lasting for more than a day even in absence of other symptoms. If the baby also has diarrhea, cold or cough than a sooner visit to the doctor is warranted.
  4. In neonates i.e. newborns a temperature lesser than 36.1°C (97°F) also warrants a visit to the doctor. In newborns thermal regulation might not be efficient and instead of rise in temperature, a decline is present.
  5. If you’re doubtful of your baby’s health, irrespective of temperature and other symptoms, you should make a call to the doctor for advice.

In children

The immune system of a child (over 2 years) has developed enough that there is no need for alarm if the child has fever as long as he/she is alert, accepting fluids and playing normally. However, the parents or guardians must look out for following signs.

  1. The child is lethargic, irritated or vomiting repetitively and also if there is severe headache, stomach ache and any other discomfort-causing signs and symptoms warrants a doctor’s help.
  2. If the child was locked inside a hot car, he/she can quickly develop a heat stroke, fever, and dehydration. Seek emergency medical care right away.
  3. The child has had the fever for more than three days.
  4. The child is lethargic, drowsy and makes little to no eye contact.
  5. In some rare cases of weak immune system or a superimposed illness, appropriate precautions (as stated by your doctor) must be taken.

In adults

In adults a doctor should be called when:

  1. The temperature is equal to 103°F (39.4°C) or more than that
  2. The fever has been persistent for three or more days

Also, immediate attention is warranted if the patient has one or more of the following signs and symptoms:

  1. Severe swelling in the throat
  2. Intolerable headache
  3. Stiffness in the neck or difficulty in bending the neck forwards
  4. An odd skin rash, particularly when it develops rapidly.
  5. Photosensitivity (i.e. abnormal sensitivity towards light)
  6. Consistent vomiting or nausea
  7. Confusion
  8. Pain in chest
  9. Difficult or painful breathing
  10. Extreme fatigue, lethargy or irritation
  11. Pain on urination
  12. Pain in the abdomen
  13. Muscular weakness or sensory problems
  14. Another unexplained signs and symptoms
  15. Seizures
The complete guide for fever: Definition, causes, symptoms and other facts 3
The Zigverve Team
The dedicated team at Zigverve that aims at bringing you the best lifestyle updates from all over the world.

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