39. Fever and sickness-behavior. The biological value of fever

Page created on March 23, 2019. Not updated since.

Biological value of fever

High temperature slows or stops the proliferation of some bacteria, but the main biological value of fever lies in how the immune system is enhanced during it. Virtually all aspects of the immune system work better in an elevated core temperature; T-cell proliferation, immunoglobulin synthesis and phagocytosis and more are more efficient.

The biological value of fever is disputed; there are down-sides to a fever as well. Despite this the evidence points to that a moderate fever is beneficial and reduces the duration of infections and the duration of excretion of the pathogen. Here are some of the unfavourable consequences of fever:

  • Potential development of hyperpyrexia
  • Potential development of febrile convulsions in children
  • The increased demand for cardiac output may cause decompensation in patients with heart failure
  • Anorexia often occurs during fever – this combined with the increased metabolic rate may cause malnutrition

Febrile convulsions or seizures are seizures that occurs in children between six months and six years that have high fever. They usually resolve spontaneously after a few minutes, but if they don’t, they should be treated with anticonvulsives and antipyretics. It very rarely has permanent consequences; the risk of developing epilepsy is less than 10%, often much lower if the convulsions are simple (consciousness is not impaired) and not complex (consciousness is impaired).

There is always the question whether taking antipyretics is a good thing or not. On one side it makes the patient feel better, but on the other side it may reduce some of the pathogen-fighting abilities of the body, potentially prolonging the infection. Taking antipyretics during an infection may prolong the infection by a few days, however in the opinion of the patient it may be better to feel slightly ill for 7 days rather than very ill for 5 days. This promotes spreading of the pathogen, however.

Sickness behaviour

Sickness behaviour is a coordinated set of behavioural changes that develop in sick individuals. They usually accompany fever. Lethargy, depression, anxiety, malaise, anorexia, sleepiness and decreased concentration are normal. At least some of these contribute to limiting the movement of the person in order to conserve energy. Anorexia works against this but may reduce ingestion of iron, which is important for bacterial reproduction. Decreased food intake reduces defecation, which reduces the possibility to spread the pathogen. Sick people are also often confined to their beds and homes and therefore have lower probability to spread the pathogen to others.


Anapyrexia is the opposite of fever, where the hypothalamus shifts the set point temperature downwards, so the person feels hot and starts to lose heat to settle at a lower core temperature. It’s not a common occurrence but it’s what causes menopausal hot flushes.

A form of anapyrexia occurs during fasting. To slow down the metabolism will the hypothalamus slightly lower the set point.

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