What is fever?

Fever in itself is not a disease. It is rather a symptom that indicates that the defence mechanism of the body is taking action against invaders causing harm to the body. Among others, viral or bacterial infections and tissue injuries can cause the body temperature to rise. If the body temperature rises above the normal body temperature range, this is called fever.

Temperature range of the body and fever
The body temperature naturally fluctuates throughout the day. For example, the body temperature is a little higher in the evening than in the morning. Thus, normal body temperature has to be considered as a temperature range. The body temperature ranges, based on rectal measurement results, can be categorized as follows:
  • Normal temperature – body temperature from 36.3 °C to 37.5 °C.
  • Increased temperature – body temperature from 37.6 °C to 38.4 °C.
  • Fever – body temperature from 38.5 °C to 38.9 °C.
  • High fever – body temperature above 39 °C.


What causes fever?
  • Viral or bacterial infections (most common cause).
  • Certain medication, seizures, cancer or autoimmune disorders.
  • Children may get low-degree fever during teething or immunizations.
Usual symptoms of a fever are:
  • Increased body temperature.
  • Headache.
  • Muscle and joint pain.
  • Chills, shaking, shivering.
  • Sweating.
  • Flushed skin.
  • Rapid heart rate.
  • Dizziness or weakness
  • With very high fever: Convulsions, hallucination or confusion

However, note that not all of the symptoms listed above may occur or appear at the same time.

Detection of fever
Taking the temperature with a fever thermometer is the most efficient and reliable way for you to detect fever.
General information on the body temperature
Your body adjusts its temperature in order to create the ideal environment for vital procedures such as the enzymatic reactions of the metabolism (enzymes need a certain surrounding temperature in order to function). Because of this, the body temperature varies up to 1 °C throughout the day. In addition, the temperature within the body (core temperature) and the surface temperature of the skin show great differences. For this reason, there is no general “normal” body temperature. As a rule, your body temperature always depends on the measurement site and is also influenced by the ambient temperature, by age, stress, duration of sleep, hormones and physical activity.



Glass thermometers and digital thermometers directly measure the body temperature, while ear and forehead thermometers determine the core temperature by measuring the infrared radiation of your body. The ear or forehead temperature reading might differ slightly from the temperature obtained by rectal, oral or axillary measurement with a digital thermometer, even if the latter measurement is carried out correctly. In the course of life, your average body temperature may decrease by up to 0.5 °C.