What Is Fever?

When one complains about feeling under the weather, the immediate action is to check for a fever. If the body temperature is higher than normal, this indicates that the individual is suffering from a fever that may have been caused by an infection. But then, what does it exactly mean when we develop a fever?

Pyrexia, also known as fever, is a  condition when the core body temperature is above the standard normal temperature of  37.2° C, set by the thermoregulatory centre in the brain that is present in the hypothalamus.1 

Fever is a defence mechanism your body uses to change the internal environment, by increasing the internal temperature in response to any trigger detected by the body. This helps the body by giving the immune system a fighting chance against the change thus allowing you to recover.2 But there is a fine line between the fever being good for the body and being bad for the body.


A healthy human body normally maintains a very stable body temperature of approximately 37 degrees Celsius (C)/ 98.6 degrees Fahrenheit (F), the stability is to the point that it varies by only 0.5° C throughout the day which shows that the body keeps a very tight control over the core temperature.3

Fevers happen because of a disturbance in the body’s sensitive workings, often the source being infections.4 The moment foreign material enters the body, it gets detected by the immune system. Foreign material can be anything ranging from a non-living material like a wood splinter, rusted nail or glass shards to a living organism such as bacteria, viruses, fungi or parasites. This triggers an internal cascade of response which results in a lot of cytokines (signalling chemical molecules) being released. 

This initial release causes two major outcomes. One is that it causes inflammation, which leads to increased blood flow in the area leading to better access for the immune cells in the blood to make it to the site of infection while simultaneously increasing the fluid pressure in the area to stop anything from escaping deeper into the body. The second outcome is that the cytokines travel in the blood for two purposes. One is that they act as a beacon for the inactive immune cells in the blood, alerting them to the start of an infection and leading them to the front lines. Second, they travel to the brain where they let the brain know that there is an infection somewhere in the body and measures need to be taken to contain the said infections.4,5

What happens next? you guessed it, a fever.

The brain signals the internal thermostat in the hypothalamus of the body, to increase the basal temperature so that the immune system gets alerted and fights better, which leads to most resources being directed to the sight of infection so that the cells fighting can divide and do whatever is necessary to contain and eradicate the infection..4 This leaves an impact on the body, resulting in the individual experiencing the ‘sick’ state of being.

When the infection is easily containable, none of these cytokines make it to the brain and thus something like a boil or pimple, acne and other forms of ailments do not trigger a fever as they are dealt with locally often resulting in inflamed pus-filled swellings. Fevers only result when the initial barrier of the immune system is breached. Below is a fever range guide.6

Low37.3 to 38.0° C (99.1 to 100.4° F)
Moderate38.1 to 39.0° C (100.6 to 102.2° F)
High39.1 to 41° C (102.4 to 105.8° F)
HyperthermiaGreater than 41° C (105.8° F)

How many types of fever are there?

The types of patterns fevers adapt and are listed in Table 1 below for reference. Each type happens with different causative agents, thus, they can be used not to identify, but to suggest a pathological condition.5

Table 1: Types of fever and their categorising conditions.5

Type of fever patternsDescription
IntermittentWhen the temperature of the body rises to above normal level and falls to normal (37.2° C) daily.The change remains between 0.3° C - and 1.4° C only.
RemittentWhen the temperature of the body rises and falls daily, however, it does not fall to normal levels and remains high.The change remains between 0.3 °C - and 1.4 °C only.
Continuous or sustainedWhen monitored for 24 hours, the change is no more than 0.3° C.
HecticThis condition is associated with the intermittent or remittent pattern, however, the key factor is that the temperature change is more than 1.4 C.
RelapsingThis is a version of the intermittent pattern with the only difference being that the spike in fever is not daily, but is separated by days or weeks of intervening normal temperatures. 

Causes of fever

Fever can be caused by a wide array of reasons. They may happen due to infections, cellular diseases like cancer or something even as unlikely as stress.7 Types of fever curves (plotting of temperature monitored over a period of time on a graph resulting in a fever curve) and their suggestive conditions are listed below in Table 2.5

Table 2: Type of fever curve and its associated suggestive pathological cause and associated symptoms (if any).

Type of fever curveSuggestive Pathological cause and associated symptoms
Continuous or sustainedInfectious: typhoid or typhus, bacterial endocarditis, tuberculosis, fungal disease, and bacterial pneumonia
Non-infectious: neoplasms, connective tissue disease, and drug fever.
HecticInfectious (Main): Abscess, Pyelonephritis, Ascending Cholangitis
Infectious (uncommon): Tuberculosis, Hypernephromas, Lymphomas and drug reaction
Associated symptoms: Chills and sweats
RelapsingMain: rat-bite fever, malaria, cholangitis, infections with Borrelia recurrentis
Non-Infectious: Hodgkin's disease (Pel-Ebstein fever), and other neoplasms.

Other than pathological conditions (such as infection and diseases), stress-related fevers can also manifest if the body is put through its paces. This develops into a psychosomatic disease and can result in individuals developing core temperatures of up to 41° C when undergoing a stressful situation. Some, on the other hand, exhibit low-grade fever that is around 37° C to 38° C, but usually, this is found in people with chronic stress in their lives (long-term stress).7

In children, fevers are common due to their developing immune system as they encounter viruses and bacteria that their body has never encountered before. Sometimes something simple like wearing warm clothes, running, or excitement can cause the core temperature of a child’s body to rise but this doesn’t necessarily mean that the child may be sick.8 Below are listed some of the most common and rarest possible causes of fever among children.

  • Infection due to Viruses or bacteria like:
    • Common cold
    • UTI (infection of the urinary tract)
    • Rubella (German measles)
    • Roseola (sixth disease)
    • Chickenpox
    • Scarlet fever
    • Mumps
    • Measles
    • Gastroenteritis
    • Ear infections
  • After a vaccination appointment: since this triggers an immune response, it also triggers the development of a fever that comes hand-in-hand with an immune response
  • Dehydration is another cause of fever as loss of fluid through sweating, vomiting, or diarrhoea can lead to dehydration leading to the development of fever
  • Heat-induced fever may happen as a result of exposure to excessive heat leading to ailments such as sunburns and sunstroke and sometimes even urticaria (hives)
  • Rare cases of diseases such as pneumonia (Lungs filling up with fluid), meningitis (Infections in the cell layer of the brain), appendicitis (infection of the appendix) or deep infections like that in the joints or bone marrow can cause the child to develop a fever
  • The rare cases can include an allergic reaction to either food or medication, rheumatic disease and metabolic diseases

Although the above-mentioned causes were listed in the children section, these may happen to adult individuals as well. An example is the Covid-19 vaccine that when administered, caused the individuals to develop a fever among other side effects.  

Signs and symptoms of fever

Below are listed some of the common signs and symptoms of fever as reported by multiple studies.9

  • Warm to the touch skin (usually the forehead, and neck)
  • Flushed skin
  • Increased heart rate (tachycardia) at rest
  • Unintended muscle contraction
  • Sweating/night sweats
  • Increased breathing rate (panting) at rest
  • Weakness
  • Dizziness
  • Nausea
  • Lightheadedness

Management and treatment for fever

The treatment of fever consists of two types, medication and environmental remedies.

A list of medications can be used to treat fevers. Most are available over the counter and do not require a prescription. Some treatments include but are not limited to the following.

  • Medicines like Ibuprofen or paracetamol in recommended doses to reduce fever
  • Drinking plenty of water and avoiding alcoholic or caffeinated drinks
  • Sponging the body to use the evaporation effect to lower the high temperature
  • Resting and avoiding stressing the body more
  • Avoid cold showers as it could cause the fever to worsen

If the condition remains unchanged and the patient is in visible discomfort, they are taken to the hospital where cold normal saline is administered to the patient (but this is in cases where medications fail to reduce the high fever).


Diagnosing a fever is very simple, easy, quick and non-invasive, especially with today’s technology. A thermometer can be used to check the body temperature. There are a multitude of different thermometers. An in-ear thermometer or a digital one is most commonly used.

Risk factors

The following list are considered risk factors:

  • Uncleanliness
  • Travelling in infection-prone areas
  • Contaminated food drinks
  • Urinary tract infection
  • Common cold
  • Seasonal flu
  • Stress  
  • Cancer 


Complications of fevers if left untreated include febrile seizures, permanent brain damage and may lead to death.10


How can I prevent fever?

Pay attention to hygiene and avoid areas where infection is common.

How common is fever?

Fever is very common in both adults and children as it is one of the most common symptoms of infections.

When should I see a doctor?

A doctor should be seen immediately in case of the following: 

  • If the fever lasts for more than three days without any improvement
  • When the fever is accompanied by a stiff neck and headache
  • If rashes don't blanch when they are pressed meaning there is internal bleeding which could be a sign of something much more seriously life-threatening


In summary, fever is a symptom, not a disease. It’s the most common symptom of infections. It’s both good and bad for us depending on the conditions. Mainly low-grade or moderate grade fever are not as dangerous as high grade fever or hyperthermia. Fever is easily treatable with over-the-counter medication and other means of cooling the body and doesn’t require much intervention unless there is a serious underlying disease that is the cause of it.


  1. Balli S, Shumway KR, Sharan S. Physiology, fever. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Jul 18]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK562334/
  2. Mackowiak PA, Chervenak FA, Grünebaum A. Defining fever. Open Forum Infect Dis [Internet]. 2021 Mar 31 [cited 2023 Nov 17];8(6):ofab161. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8394829/
  3. Del Bene VE. Temperature. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations [Internet]. 3rd ed. Boston: Butterworths; 1990 [cited 2023 Jun 8]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK331/
  4. Wrotek S, LeGrand EK, Dzialuk A, Alcock J. Let fever do its job. Evol Med Public Health [Internet]. 2020 Nov 23 [cited 2023 Jul 20];9(1):26–35. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7717216/
  5. Dall L, Stanford JF. Fever, chills, and night sweats. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations [Internet]. 3rd ed. Boston: Butterworths; 1990 [cited 2023 Jun 8]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK324/
  6. Islam MA, Kundu S, Alam SS, Hossan T, Kamal MA, Hassan R. Prevalence and characteristics of fever in adult and paediatric patients with coronavirus disease 2019 (COVID-19): A systematic review and meta-analysis of 17515 patients. PLoS One [Internet]. 2021 Apr 6 [cited 2023 Jul 20];16(4):e0249788. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8023501/
  7. Oka T. Psychogenic fever: how psychological stress affects body temperature in the clinical population. Temperature (Austin) [Internet]. 2015 Jun 3 [cited 2023 Jun 8];2(3):368–78. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4843908/
  8. Fever in children: Overview [Internet]. Institute for Quality and Efficiency in Health Care (IQWiG); 2019 [cited 2023 Jun 8]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279455/
  9. Ames NJ, Powers JH, Ranucci A, Gartrell K, Yang L, VanRaden M, et al. A systematic approach for studying the signs and symptoms of fever in adult patients: the fever assessment tool (Fast). Health Qual Life Outcomes [Internet]. 2017 Apr 27 [cited 2023 Jul 20];15:84. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5408372/
  10. Walter EJ, Hanna-Jumma S, Carraretto M, Forni L. The pathophysiological basis and consequences of fever. Crit Care [Internet]. 2016 [cited 2023 Jul 20];20:200. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4944485/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Ayesha Jamil

Bachelors of Biomedical Sciences with Honors - University of Nottingham, Malaysia

Ayesha is a passionate graduate who is highly motivated to do her best in all that she tries. functioning in a multicultural university society on campus, she has developed many skills including communication, leadership skills, and developed a strong work ethic to name a few while in university.

She was the course representative for all three of her academic years working closely with the teaching and management staff to ensure the needs and voices of her fellow students were heard as well as the implementation of any new protocol for the students that may have been introduced for them.

She has also undertaken many key roles as well as been a part of the volunteering fellows. She has taken another step towards the future by undertaking an internship with Klarity as a writer for medical articles.

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