Description of the physiological process of fever

Fever is an evolutionary, sophisticated, precisely targeted and intricately regulated, healthy, defensive response of the human body that is beneficial for the healing and survival of the host.

Fever does not protect the body alone, but in combination with many other processes.

For this long-term protection, the body intentionally accepts temporary, slight disadvantages. Just like we do during a workout.

Raising body temperature is an energy- and fluid-intensive process. An increase of 1°C requires a 10-12% increase in metabolism and a 15-20% increase in fluid demand (2). See further in the chapter „Benefits of Fever.”

Although in practice it has not yet been implemented in many places (16); there is general agreement that the potential benefits of fever need to be weighed against its slightly uncomfortable, slightly exhausting effects (17, 123).


The definition of fever

Interestingly, fever does not have a commonly accepted definition worldwide. Reviewing the official professional protocols in 150 countries, the concept of fever is not uniform.

Normal body temperature ranges from 36°C to 37.5°C. Therefore, some countries like Japan are already talking about fever at temperatures above 37.5°C. In fact, rectal morning temperature is considered to be elevated body temperature above 37.2°C and evening temperature above 37.7°C (20). The German Paediatric Society defines fever as a core temperature above 38.5°C. Temperatures between 37.5 and 38.5°C are referred to as "raised temperature". Many protocols already call temperatures "fever" above 38.0°C (17, 21-35).


According to current professional recommendations (123):

For values measured under the arm (axillary temperature)(without subtraction or addition):

36-37°C normal body temperature

37-38°C rise in temperature (subfebrile)

38-39°C moderate fever

39-40.5°C high fever

Above 40.5°C very high fever


For rectal values, half a degree higher, i.e. (without subtraction or addition):

36.5-37.5°C normal body temperature

37.5-38.5°C rise in temperature (subfebrile)

38.5-39.5°C moderate fever

39.5-41°C high fever

Above 41°C very high fever


How does fever develop?

Fever is produced by increasing heat production and reducing heat dissipation at the same time.

The former is caused by accelerated muscle metabolism, the latter is caused by a decrease in blood circulation to the skin. Initially, as the fever rises, this causes children to become pale, to shiver, and for their skin to become cold. In extreme circumstances literally shivers or trembling of the muscles. This condition lasts until the fever reaches the body’s set-point.


What triggers a fever reaction? What do we respond to with fever?

There are various reasons for this.

1) Physical causes:

  • common cold
  • infection (virus and/or bacteria, other pathogens)
  • teething (especially during the eruption of the front ones at 6 to 8 months of age)
  • vigorous physical activity
  • heat stroke, sunstroke
  • poisoning

2) Psycho-social reasons:

In children, both positive and negative experiences can cause elevated temperatures. For example: preparing for an exciting event, a birthday party, a long trip, a family quarrel or a trauma.

The optimal temperature for the growth of viruses and bacteria is 33-35°C, which means a cold in humans. From this point of view, too, we should speak of a genuine "cold or chill".

Accordingly, there is an optimum fever (usually 38-40°C) where the body is most effective in killing viruses and bacteria, preventing them from reproducing and activating the immune system.


About the course of fever and its stages, see that separate entry.


You can find the corresponding numbered references here: References

Version update: 1 March 2024