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What is considered a fever?

Fever is considered the increase in the core body temperature above normal. The normal body temperature, regulated by the hypothalamus (a region in the brain), is usually around 37 degrees Celsius. Normal body temperatures can fluctuate throughout the day, depending on factors such as age, time of day, activity level, eating habits, sleeping or waking habits and changes in hormone levels.

Fever generally occurs due to infections or other medical conditions such as inflammatory diseases. It is one of the most common symptoms of infections found in children. 

How is fever classified or identified?

There are several ways to determine what is considered a fever based on factors such body temperature and duration of the elevated body temperature.

A fever can be classified, based on body temperature, as low grade, moderate grade or high grade. Individuals with elevated body temperatures ranging approximately between 38 degrees Celsius (100.4°F) and 39 degrees Celsius is considered to have low grade fever. Those with body temperatures above 40 degrees Celsius (104°F) are considered to have high fever.

Fever can also be classified as acute or chronic, based on its duration. Acute fevers last less than seven days, while chronic fevers usually have a duration of over two weeks and are linked to serious medical conditions such as Tuberculosis, HIV, cancer and chronic inflammatory disorders.

Fevers can also have different patterns. These can include continuous or sustained fever, intermittent fever, remittent fever and relapsing fever.  

Patients with continuous fever experience small fluctuations in their body temperature, however the temperature stays above normal. Continuous fever is typically linked to medical illnesses such as typhoid, pneumonia, bacterial meningitis and urinary tract infection.

Patients with intermittent fever generally experience the fever for several hours in a day. Those with remittent fever experience changes in their body temperature, which remains above normal. Relapsing fevers are periodic, alternating between low grade and no fever.

How does fever fight infections?

A fever is generally induced by the body’s immune system as a response to infections or inflammatory disorders. This response is initiated by substances known as pyrogens, which originate from inside and outside the body.

Pyrogens, formed outside the body, are toxins released by microorganisms responsible for causing infections. Those formed within the body are cytokines, which are produced by immune cells in response to an infection or other illnesses.

Fevers help enhance the immune system by stimulating the production of white blood cells – the immune cells responsible for fighting off infections. The increased temperatures associated with fevers prevent the growth of the microorganisms (bacteria or virus) that cause the infections.

While some studies suggest that fever is linked to increased chances of survival through its response in fighting off infections, according to studies, high fever is linked to serious or life-threatening medical conditions such as sepsis, neurological injuries, or cardiopulmonary disease. Substances known as cryogens help prevent the body temperatures from increasing too much. This is done by blocking the synthesis of the pyrogens.

Signs and symptoms of fever

A common sign associated with fever is the elevation of body temperature. A temperature above normal is usually considered a sign that you may have a fever. Body temperature can be measured in the mouth, ear, rectum, under arm and the skin. According to clinical studies, rectal and oral temperature measurements are considered to be some of the most accurate methods in determining the body temperature.

Other common signs and symptoms of fever can include headache, fatigue, discomfort, dehydration, loss of appetite, muscle aches and sweating. The body responds to fever by decreasing the blood flow in the skin, preventing heat loss. This results in the individual experiencing symptoms such as chills and shivering.

If you are experiencing any serious signs or symptoms such as increased heart rate or shortness of breath, contact your doctor and seek medical attention.

Children under five years may be at risk of developing febrile seizures. The seizures are a neurological disorder commonly affecting young children. Febrile seizures can occur because of fever or elevation in body temperature as a result of an infection. The seizures usually last around a few minutes.  They generally occur within a day after getting a fever.

Treating and preventing fever

Treatment or management of symptoms associated with fever usually depends on the patient’s age and medical condition. Patients with mild or low-grade fever can manage their symptoms with a few home remedies. These remedies include rest and drinking plenty of fluids.

Patients can be treated with medications known as antipyretics that can help reduce the fever. These medications include acetaminophen and ibuprofen and can be obtained over the counter.

If you or your child have high fever, then you should contact your doctor and seek medical attention. During the medical visit, the doctor will collect your health information and then perform a physical examination. The health information can include your symptoms, medical condition or any medications taken or currently taking.

The doctor may inquire if you have recently travelled or have exposure to anyone with infections. Your doctor usually looks for symptoms such as sore throats, pain, or nasal congestion to help assess the cause of your fever.



Ogoina D. Fever, fever patterns and diseases called ‘fever’–a review. J Infect Public Health. 2011 Aug;4(3):108-24. doi: 10.1016/j.jiph.2011.05.002. Epub 2011 Jun 14. PMID: 21843857.

Balli S, Sharan S. Physiology, Fever. [Updated 2020 Sep 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562334/

Barbi E, Marzuillo P, Neri E, Naviglio S, Krauss BS. Fever in Children: Pearls and Pitfalls. Children (Basel). 2017;4(9):81. Published 2017 Sep 1. doi:10.3390/children4090081

Evans SS, Repasky EA, Fisher DT. Fever and the thermal regulation of immunity: the immune system feels the heat. Nat Rev Immunol. 2015;15(6):335-349. doi:10.1038/nri3843

Ames NJ, Powers JH, Ranucci A, 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. 2017;15(1):84. Published 2017 Apr 27. doi:10.1186/s12955-017-0644-6

Leung AK, Hon KL, Leung TN. Febrile seizures: an overview. Drugs Context. 2018;7:212536. Published 2018 Jul 16. doi:10.7573/dic.212536

Mosili P, Maikoo S, Mabandla MV, Qulu L. The Pathogenesis of Fever-Induced Febrile Seizures and Its Current State. Neurosci Insights. 2020;15:2633105520956973. Published 2020 Nov 2. doi:10.1177/2633105520956973

Guppy MP, Mickan SM, Del Mar CB, Thorning S, Rack A. Advising patients to increase fluid intake for treating acute respiratory infections. Cochrane Database Syst Rev. 2011;2011(2):CD004419. Published 2011 Feb 16. doi:10.1002/14651858.CD004419.pub3

Fever in Adults. Merck Manual Professional Version. Retrieved from https://www.merckmanuals.com/home/infections/biology-of-infectious-disease/fever-in-adults.

Allan GM, Arroll B. Prevention and treatment of the common cold: making sense of the evidence. CMAJ. 2014;186(3):190-199. doi:10.1503/cmaj.121442

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