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Why! the fever itself is nature's instrument.

Thomas Sydenham (1624–89), Medical Observations


Although fever is a sign of disease and usually occurs in response to infection (mainly viral), its presence is recognised as playing an important role in the individual's defence against infection. The infecting pathogen triggers hypothalamic receptors, causing the thermostatic mechanisms to be reset to maintain core temperature at a higher level. The elevation in temperature results from increased heat production (e.g. shivering) or decreased loss (e.g. peripheral vasoconstriction). The elevation in body temperature activates T-cell production, increases the effectiveness of interferons and limits the replication of some common viruses.1


Key facts and checkpoints

  • Fever plays an important physiological role in the defence against infection.

  • Normal body temperature (measured orally mid-morning) is 36–37.2°C (average 36.8°C).

  • Fever can be defined as an early morning oral temperature >37.2°C or a temperature >37.8°C at other times of day.2

  • Oral temperature is about 0.4°C lower than core body temperature.

  • Axillary temperature is 0.5°C lower than oral temperature.

  • Rectal, vaginal and ear drum temperatures are 0.5°C higher than oral and reflect core body temperature.

  • There can be a normal diurnal variation of 0.5–1°C (lowest in early morning and highest in late afternoon).

  • Fevers due to infections have an upper limit of 40.5–41.1°C (105–106°F).

  • Hyperthermia (temperature above 41.1°C) and hyperpyrexia appear to have no upper limit.

  • Infection remains the most important cause of acute fever.3

  • Symptoms associated with fever include sweats, chills, rigors and headache.

  • General causes of fever include infections, malignant disease, mechanical trauma (e.g. crush injury), vascular accidents (e.g. infarction, cerebral haemorrhage), immunogenic disorders (e.g. drug reactions, SLE), acute metabolic disorders (e.g. gout), and haemopoietic disorders (e.g. acute haemolytic anaemia).3

  • Drugs can cause fever, presumably because of hypersensitivity.3 Important examples are allopurinol, antihistamines, barbiturates, cephalosporins, cimetidine, methyldopa, penicillins, isoniazid, quinidine, phenolphthalein (including laxatives), phenytoin, procainamide, salicylates and sulphonamides.

  • Drug fever should abate by 48 hours after discontinuation of the drug.4

  • Infectious diseases at the extremes of age (very young and aged)3 often present with atypical symptoms and signs. Their condition may deteriorate rapidly.

  • Overseas travellers or visitors may have special, even exotic infections and require special evaluation (refer to CHAPTER 15).

  • Immunologically compromised patients (e.g. AIDS patients) pose a special risk for infections, including opportunistic infections.

  • A febrile illness is characteristic of the acute infection of HIV: at least 50% have an illness like glandular fever. Think of it!


The abrupt onset of fever with a chill or rigor is a feature of some diseases. Examples include:


  • bacteraemia/septicaemia

  • pneumococcal pneumonia

  • pyogenic infection with bacteraemia

  • lymphoma

  • pyelonephritis

  • visceral abscesses (e.g. perinephric, lung)

  • malaria

  • biliary sepsis (Charcot triad—jaundice, right hypochrondial pain, fever/rigors)


Features of a true chill are teeth chattering and bed shaking, which is quite different from the chilly sensations that occur ...

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