Hanna Rutkowski is a freelance writer for the NetDoktor medical editorial team.

Lisa Vogel studied departmental journalism with a focus on medicine and life sciences at Ansbach University of Applied Sciences and deepened her journalistic knowledge in the master’s program in multimedia information and communication. This was followed by a traineeship in the NetDoktor editorial office. Since September 2020, she has been writing as a freelance journalist for NetDoktor.

Martina Feichter studied biology with pharmacy as an optional subject in Innsbruck and also delved into the world of medicinal plants. From there it was not far to other medical topics that still fascinate her today. She trained as a journalist at the Axel Springer Akademie in Hamburg and has worked for NetDoktor since 2007 – first as an editor and since 2012 as a freelance writer.

At Fever If the body temperature is elevated above normal levels. With this heating up, the body defends itself against harmful influences such as invading pathogens. But malignant tumors and inflammatory autoimmune diseases also produce fever. Learn here when fever is present, how exactly it occurs, what causes it can have and what you can do about it.


Brief overview

  • At what point does one have a fever? When the body temperature rises above 38 degrees Celsius.
  • Possible accompanying symptoms: Dry and hot skin, shiny eyes, chills, loss of appetite, accelerated breathing rate, restlessness, confusion, hallucinations, etc.
  • Causes: very diverse, u.a. Infections (flu, pneumonia, tuberculosis, covid-19, tonsillitis, measles, blood poisoning, etc.).), pus collections (abscesses), appendicitis, renal pelvic inflammation, heart valve inflammation, rheumatic diseases, inflammatory bowel disease, stroke, tumors, etc.
  • When to see a doctor? Adults: when the fever is high, prolonged or recurrent. Children: when the fever lasts longer than a day, from other complaints (drowsiness, rash, vomiting, etc.) to the symptoms.) is accompanied, fever-reducing measures do not help or a febrile convulsion occurs. Infants: at temperatures above 38 degrees Celsius.
  • Treatment: Home remedies (drink a lot, compress the calf, lukewarm bath, etc.).), antipyretic drugs, treatment of the underlying disease

Fever: description

Basically, fever is nothing threatening: the rise in core body temperature is rather a battle cry against harmful influences and threats. In fact, certain defense mechanisms run faster in warmer environments:

For example, when pathogens such as bacteria, parasites or viruses invade the body, the body activates its defense system. Various substances are released during this process, including so-called Pyrogenic. These are substances (such as cellular debris, toxins, etc.), which give the signal in the brain to heat up. During the work of the immune system against malignant tumors (cancer) as well as during autoimmune diseases, pyrogens can be released which cause fever.

To be distinguished from fever is hyperthermia (overheating). The body temperature is also elevated, but not by pyrogens. The best-known example of hyperthermia is heat stroke or sunstroke – a very warm environment leads to a misregulation of the heat center in the brain. antipyretic drugs do not help against hyperthermia.

When does one have a fever?

Normal body temperature not only varies from person to person, but is also subject to its own daily rhythm. It is lowest at night around two o’clock, but then rises slowly before waking up, reaching its peak in the afternoon. Fluctuations of more than one degree may occur. On average normal body temperature between 36.0 and 37.4 degrees Celsius (measured rectally). But even here, depending on the accuracy of the measurement method, slightly different values are sometimes given.

In women, body temperature rises by about 0.5 degrees Celsius during ovulation and pregnancy.

If the body temperature rises above the normal level, physicians distinguish the following gradations:

  • Elevated temperature (subfebrile): temperatures between 37.5 and 38 degrees Celsius are called subfebrile. Possible causes are infections caused by bacteria or viruses, but also heatstroke or intensive sports.
  • slight fever: 38 degrees Celsius is the limit of fever. A slight fever is present with readings between 38.1 and 38.5 degrees Celsius.
  • moderate feverTemperatures between 38.6 and 39 degrees Celsius are considered moderate fever.
  • high feverHigh fever is defined as a reading between 39.1 and 39.9 degrees Celsius.
  • very high feverA body temperature of more than 40 degrees Celsius is usually fatal.
  • extreme fever (hyperpyrexia): Natural fever rarely reaches values above 41 degrees Celsius. From 41.1 one speaks of hyperpyretic fever.

Very high and extreme fever can cause tissue and organ damage and thus become dangerous. A body temperature of over 42.6 degrees Celsius is usually fatal.

Signs of fever

tiredness, fatigue, headache, sensitivity to light and sound, general feeling of illness – fever is often accompanied by various complaints. Appetite also often decreases, muscles and joints ache, and all you want to do is rest in bed. Depending on the severity of the fever, the following signs are also typical:

  • dry, hot skin
  • "feverish" shining eyes
  • in children: Quivering
  • Feeling of thirst and heavy sweating
  • during the temperature rise but shivering to chills
  • sometimes digestive problems with loss of appetite, diarrhea or vomiting
  • faster breathing
  • Restlessness, confusion

Infants sometimes do not develop fever even in severe infections. In them, therefore, attention should be paid to other possible signs of illness, such as listlessness, conspicuous sleepiness, refusal to drink, repeated vomiting, diarrhea, unusual skin color or rash. In the case of such symptoms, the pediatrician should be consulted.

How does fever arise?

The body temperature is controlled in the brain: the hypothalamus is where the body’s Heat regulation center. Via cold and heat sensors in the skin and body, it receives information about the ambient and organ temperature.

If it becomes too warm inside the body, the organism can take countermeasures by dilating the blood vessels of the skin and "ordering" increased sweating. If, on the other hand, the body becomes too cool, the skin vessels constrict and goose bumps form, both of which help to reduce the release of heat. At the same time, heat production is increased – through muscle tremors ("shivering from cold") and increased metabolism.

Since fever is also mediated by the same brain center, the typical symptoms can be explained: If the body is supposed to fever (to fight pathogens, for example), on the one hand the heat output is reduced. In addition, the skin vessels are constricted, making the skin pale and cold. The body sweats less, which helps heat up inside. In addition, the metabolism is boosted and muscle tremors (chills) are triggered – this increases heat production.

When the hypothalamus tells the fever to go down, the peripheral vessels dilate – the skin becomes warm and reddened. In addition, the patient begins to sweat. Excess heat is dissipated via both mechanisms and the body is thus cooled.

Fever: Types of progression

Medical experts distinguish between different temperature courses in fever:

  • Continuous feverThe temperature remains elevated at approximately the same level for more than four days, reaching values of over 39 degrees Celsius and fluctuating by no more than one degree during the day. This course often occurs with bacterial infections such as scarlet fever, typhoid fever or bacterial pneumonia.
  • Remitting feverThe patient has a fever practically all day, but less in the morning than in the evening (the difference is one to two degrees). A remittent fever is seen in some viral infections, tuberculosis, bronchitis, pus accumulation and rheumatic fever.
  • Intermittent feverIn this case, the fever fluctuates even more during the course of the day. The body temperature is (approximately) normal in the morning and then rises to sometimes high fever levels by the evening (the difference is more than two degrees Celsius). This can be observed, for example, in pleurisy, sepsis, salmonellosis, endocarditis and osteomyelitis. Tumor diseases (such as Hodgkin’s disease) can also trigger an intermittent fever.
  • Undulant fever: An undulating (undulating) course of fever may occur, for example, in brucellosis. In lymphomas (such as Hodgkin’s disease), fever can also be undulating: In this case, fever phases lasting several days alternate with fever-free phases of approximately the same length. Doctors refer to this as Pel-Ebstein fever.
  • Recurrent feverOne speaks of a recurring (or recurrent) fever when there are regularly one or two (occasionally up to 14) fever-free days between individual episodes of fever. Such a course is typical for malaria. A recurrent fever can also occur with certain bacterial infections. An example of this is the five-day fever in an infection with Bartonella bacteria.
  • Two-peaked (biphasic) feverAfter a few days of fever, the temperature drops back to normal, before a second fever phase of several days follows. Such a two-peaked fever curve may occur, for example, in measles or meningococcal blood poisoning (meningococcal sepsis).

How to measure fever?

The data on average body temperature are all a little inaccurate. The reason is that body temperature is not only influenced by the time of day, activity and individual variations, but also depends on the method of measurement. The type and location of the measurement affect (slightly) the result of the measurement:

  • Taking a fever in the anus (rectally): The measurement with the thermometer in the buttocks is the most unpleasant, but also the most accurate method. So the measurement result obtained is closest to the temperature inside the body.
  • Taking a fever under the tongue (sublingual)If the thermometer is placed under the tongue, good results are also obtained. These are usually about 0.3 degrees lower than the rectal measurement. When using the sublingual method, make sure that the lips are closed during the measurement (sometimes difficult if the nose is stuffy!). In addition, the patient must not have eaten or drunk anything cold or warm before the measurement, because otherwise the result would be falsified.
  • Taking a fever in the ear (auricular): Taking a temperature using infrared waves in the ear is useful, especially for children, because it is quick and easy. For this purpose, the probe must be inserted into the ear canal, the easiest way being to gently pull on the back of the ear. In case of middle ear infection and other ear diseases it is better to take the temperature in the healthy ear.
  • Taking a temperature under the armpit (axillary): It is the most popular, but least accurate method of taking a temperature. The measured temperature value can be up to 0.5 degrees below the actual temperature inside the body.

Fever: causes and possible diseases

Best known is the symptom fever with a flu or a strong cold. In fact, infections with pathogens are the most common reason for a pathologically elevated body temperature. Sometimes, however, non-infectious diseases such as appendicitis, rheumatism or cancer are behind it. So there are many possible causes of fever. Here are some common examples:

  • Colds (flu) and influenza
  • Pneumonia (e.g. caused by pathogens like pneumococcus)
  • Covid-19
  • Infections with streptococci such as purulent tonsillitis, blood poisoning (sepsis) or inflammation of the inner wall of the heart (endocarditis)
  • Renal pelvic inflammation
  • purulent abscesses
  • Appendicitis
  • Tuberculosis
  • vascular inflammation (vasculitis)
  • Diseases of the connective tissue (collagenoses)
  • Lymph gland cancer (non-Hodgkin lymphoma, Hodgkin lymphoma = Hodgkin’s disease) and other tumors
  • rheumatic diseases (ankylosing spondylitis, rheumatoid arthritis, systemic lupus erythematosus, etc).)
  • chronic inflammatory bowel diseases (Crohn’s disease, ulcerative colitis)
  • chronic or alcohol-related liver inflammation (hepatitis)
  • Hormonal disorders (Addison’s crisis, inflammation of the parathyroid gland, etc.) are not to be avoided.)
  • Blockage of a blood vessel by a blood clot (thrombosis)
  • Allergies


Fever of unknown origin (FUO)

Of fever of unknown origin ( fever of unknown origin ), FUO) is what doctors call it when a patient’s body temperature remains above 38.3 degrees Celsius for more than three weeks and no explanation can be found using standard tests.

In many patients the cause of the FUO is found in the end. Often, these have been undiscovered infections, tumor diseases, rheumatic diseases or autoimmune diseases. Also Drugs can trigger a fever of unknown cause: For example, some people are hypersensitive to certain active ingredients in diuretics (diuretics), painkillers, antibiotics, sleeping pills or sedatives. The fever can be an expression of this "hypersensitivity" its.

Often one observes a fever of unknown cause moreover in HIV patients. The trigger is often pathogens that normally – that is, in people with healthy immune systems – do not cause infection.

Causes of fever in children

Children suffer from fever more often than adults. In most cases, even small infections are enough to raise the temperature in them. The guideline for when a fever is present is defined in the same way as for adults. It is at 38 degrees Celsius.

The most common causes of fever in children are otitis media, gastrointestinal infections, bacterial respiratory infections with tonsillitis, coughs, colds and sore throats. Often the small fever patients also suffer from a typical children’s disease such as scarlet fever, measles or the three-day fever. In rare cases, a severe bacterial infection (such as pneumonia, meningitis) or a rheumatic disease is the reason for a pathologically elevated body temperature.

Diseases with this symptom

Find out here about the diseases in which the symptom can occur:

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