International Classification (ICD) R50.-


Fever refers to an increase in normal body temperature and is not normally a disease in its own right. Rather, fever is understood as a body reaction to external (for example, infections) or internal (such as tumors) influences. Thus, fever supports the immune system in its function.

As a rule, one can assume that the body temperature in the morning is about 36.5°C (rectal), 32.2°C (oral) and 36.0°C (axillary). During the day, the temperature increases by about one degree Celsius until it finally reaches its maximum value (maximum) in the late afternoon.

It should be noted, however, that the above values are only average values. The body temperature can show strong fluctuations in different (healthy) people. In addition, many factors, such as stress, exercise, caffeine, diet, and certain medications, can affect temperature. M

ou can distinguish between different types of fever:

  • Elevated temperature (no fever yet): below 38°C (measured rectally).
  • Moderate fever: up to 39°C (measured rectally)
  • High fever: over 39°C (measured rectally)


Normally, the body temperature is subject to the control of a certain part of the brain, the hypothalamus, which also has the task of keeping the temperature within normal limits. If there is a fever, the hypothalamus increases the reference value and the body temperature rises as a result.

Apart from this, there are also certain substances that can trigger fever. These include components of carcinogens, which doctors call pyrogens. The most common causes of fever include:

  • Infections triggered by viruses, bacteria or parasites (e.g. flu or malaria).
  • Medication
  • Increased metabolic activity (e.g. in hyperthyroidism)
  • Physical exertion
  • Heat stroke
  • Extreme fluid loss
  • Operations (even without wound infection)
  • Malignant tumours
  • Inflammatory autoimmune diseases of the connective tissue (collagenosis) and the vessels (vasculitides)

In the period after ovulation in women, the body temperature rises by about 0.5°C. In some individuals, no cause for the increased temperature or fever can be determined. In this case, doctors speak of an "unclear genesis".


Depending on the level and cause of the fever, the following other symptoms may occur in addition to the increased temperature:

  • Fatigue, weakness, drowsiness, feeling sick.
  • aching limbs and joints
  • loss of appetite
  • shivering and chills as soon as the fever rises
  • Sweating, also with heavy loss of fluids, when the fever drops
  • Warm, red skin, dry lips and oral mucosa
  • Rapid heartbeat (tachycardia)
  • Rapid, shallow breathing


To resolve the fever, the underlying disease must be treated. Diagnosis includes:

  • Taking the medical history (anamnesis). Here, also stays abroad, contacts to other sick persons or sick animals must be inquired.
  • A detailed physical examination
  • Measurement of the body temperature under the armpit (axillary), as well as in the anus (rectal).
  • A blood sample is taken, as this can also provide important information about the cause of the disease.

Further examinations are useful to find the cause of the fever:

  • The sputum, urine, stool, blood, or other body fluids are examined for other bacteria and pathogens
  • X-rays of the lungs or sinuses are taken
  • Examination of the abdomen, heart and/or thyroid gland with ultrasound (sonography)
  • endoscopy of the gastrointestinal tract
  • Examination by gynaecologist, urologist, dentist and ENT specialist


Since fever is part of the body's defense against infections, it is in no case sensible to lower the fever immediately. However, it is still important that the doctor prescribes an appropriate therapy and finds out the cause of the illness.

  • If the affected person has a body temperature of over 39°C and complains of a severe feeling of illness or strain on the cardiovascular system, the doctor will give fever-reducing medication such as paracetamol.
  • If it is a bacterial infection, antibiotics may be given

What you can do yourself:

  • If the fever lasts for several days and the temperature is very high (over 39°C) with a strong feeling of illness, the cause should be clarified by a doctor
  • Rest and bed rest is important until the fever has subsided.
  • Drink enough, as a large amount of fluid is lost due to the fever. As a rule of thumb: If the temperature rises by one degree Celsius above 37°C, 0.5 to one litre of fluid should be added.
  • If necessary, fever-reducing methods can be used, such as cold compresses, vinegar foot compresses or wet and cold calf compresses.


Normally, fever passes without complications. However, the underlying disease is also decisive for this:

  • If the fever lasts for a long time and is not treated, the body is severely weakened by the constant loss of fluids and the increased basal metabolic rate.
  • Without medical treatment, a bacterial infection can easily turn into blood poisoning (sepsis).
  • In the case of children under 5 years of age, additional care must be taken to ensure that they do not develop febrile convulsions.

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All information used for the content comes from verified sources (recognised institutions, experts, studies by renowned universities). We attach great importance to the qualification of the authors and the scientific background of the information. Thus, we ensure that our research is based on scientific findings.
Danilo Glisic

Danilo Glisic

As a biology and mathematics student, he is passionate about writing magazine articles on current medical topics. Due to his affinity for facts, figures and data, his focus is on describing relevant clinical trial results.

The content of this page is an automated and high-quality translation from DeepL. You can find the original content in German here.



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