fever

and Lisa Vogel, medical editor and Martina Feichter, medical editor and biologist

Hanna Rutkowski is a freelance writer for the medical team.

More about the experts

Lisa Vogel studied departmental journalism with a focus on medicine and biosciences at Ansbach University and deepened her journalistic knowledge in the master's degree in multimedia information and communication. This was followed by a traineeship in the editorial team. Since September 2020 she has been writing as a freelance journalist for

Brief overview

When 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, including infections (flu, pneumonia, tuberculosis, Covid-19, tonsillitis, measles, blood poisoning, etc.), accumulations of pus (abscesses), appendicitis, pelvic inflammation, heart valve inflammation, rheumatic diseases, inflammatory bowel disease, stroke, etc. .
When to the doctor Adults: if the fever is high, persistent or recurring. Children: if the fever lasts longer than a day, is accompanied by other symptoms (drowsiness, rash, vomiting, etc.), fever-lowering measures do not help or a febrile seizure occurs. Infants: at temperatures above 38 degrees Celsius.
Treatment: home remedies (drink a lot, leg compresses, lukewarm bath, etc.), antipyretic medication, treatment of the underlying disease

Fever: description

Basically, fever is not threatening: the rise in core body temperature is rather a declaration of war against harmful influences and threats. Certain defense mechanisms run faster in a warmer environment:

For example, when pathogens such as bacteria, parasites or viruses enter the body, the body activates its defense system. Various substances are released, including so-called pyrogens. These are substances (such as cell debris, toxins, etc.) that give the brain the signal to heat up. Fever-inducing pyrogens can also be released during the work of the immune system against malignant tumors (cancer) and with autoimmune diseases.

Hyperthermia (overheating) must be distinguished from fever. The body temperature is also increased, but not due to pyrogens. The best-known example of hyperthermia is heat stroke or sunstroke - a very warm environment leads to a dysregulation of the heat center in the brain. Antipyretic drugs do not help against hyperthermia.

When does one have a fever?

The normal body temperature not only fluctuates from person to person, but is also subject to its own daily rhythm. It is lowest at around two o'clock in the morning, but then increases slowly before you wake up, and then reaches its highest level in the afternoon. This can lead to fluctuations of more than one degree. On average, the normal body temperature is between 36.0 and 37.4 degrees Celsius (measured rectally). But even here, depending on the accuracy of the measurement method, some slightly different values are given.

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

If the body temperature rises above the normal level, doctors differentiate between the following levels:

Elevated temperature (subfebrile): Temperatures between 37.5 and 38 degrees Celsius are referred to as subfebrile. Possible causes are infections caused by bacteria or viruses, but also heat stroke or intense exercise.
mild 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 fever: Temperatures between 38.6 and 39 degrees Celsius are considered to be moderate fever.
high fever: If the reading is between 39.1 to 39.9 degrees Celsius, there is a high fever.
Very high fever: This means a body temperature of more than 40 degrees Celsius.
Extreme fever (hyperpyrexia): Natural fever rarely reaches values above 41 degrees Celsius. From 41.1 one speaks of hyperpyretic fever.

Very high and extreme fevers can damage tissues and organs, making them dangerous. A body temperature of over 42.6 degrees Celsius is usually fatal.

Signs of fever

Fatigue, exhaustion, headache, sensitivity to light and noise, general feeling of illness - fever is often accompanied by various symptoms. The appetite also often decreases, the muscles and joints ache and you just want to have some rest in bed. Depending on the severity of the fever, the following signs are also typical:

dry, hot skin
"Feverish" shining eyes
in children: whining
Feeling thirsty and sweating profusely
but during the rise in temperature shivering to the point of chills
sometimes indigestion with loss of appetite, diarrhea or vomiting
faster breathing
Restlessness, confusion

Infants sometimes do not develop a fever even with severe infections. You should therefore watch out for other possible signs of illness such as indifference, noticeable sleepiness, refusal to drink, repeated vomiting, diarrhea, unusual skin color or rash. In the presence of such symptoms, one should consult the pediatrician.

How does a fever develop?

The body temperature is controlled in the brain: the heat regulation center is located in the so-called hypothalamus. It receives information about the ambient and organ temperature via cold and heat sensors in the skin and in the body.

If it becomes too warm inside the body, the organism can counteract this by expanding the blood vessels in the skin and "arranging" increased sweating. If, on the other hand, it becomes too cool for the body, the skin vessels constrict and goose bumps form, both of which help to reduce the heat output. At the same time, the heat production is increased - through muscle tremors ("trembling from the cold") and an increased metabolism.

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

When the hypothalamus commands the fever to decrease, the peripheral vessels widen - 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: progressive forms

Doctors differentiate between different temperature profiles in fever:

Continuous fever: The temperature remains roughly the same for longer than four days, reaches values of over 39 degrees Celsius and fluctuates by a maximum of one degree during the day. This course often occurs with bacterial infections such as scarlet fever, typhoid, or bacterial pneumonia.
Remitting fever: The patient has a fever practically the whole day, but less in the morning than in the evening (the difference is one to two degrees). A remitting fever shows up with some viral infections, tuberculosis, bronchitis, accumulations of pus and rheumatic fever.
Intermittent fever: Here the fever fluctuates even more clearly during the day. The body temperature is (almost) normal in the morning and then rises to sometimes high fever values by the evening (the difference is more than two degrees Celsius). This can be observed, for example, in pleurisy and pleurisy, blood poisoning (sepsis), salmonella infection (salmonellosis), heart inflammation (endocarditis) and inflammation of the bone marrow (osteomyelitis). Tumor diseases (such as Hodgkin's disease) can also trigger an intermittent fever.
Undulating fever: An undulating (undulating) fever can occur, for example, in brucellosis. Even with lymphoma (such as Hodgkin's disease), the fever can run undulating: Several days of feverish phases alternate with fever-free phases of about the same length. Doctors speak of Pel-Ebstein fever.
Recurrent fever: One speaks of a recurrent (or recurrent) fever if there are regularly one or two (occasionally up to 14) fever-free days between individual fever attacks. Such a course is typical of malaria. A recurrent fever can also occur with certain bacterial infections. An example of this is the five-day fever when infected with Bartonella bacteria.
Two-peaked (biphasic) fever: After a few days of fever, the temperature drops back to normal before a second feverish phase of several days follows. Such a two-peak fever curve can occur, for example, with measles or blood poisoning from meningococci (meningococcal sepsis).

How to measure a fever?

The average body temperature information is all a little inaccurate. The reason is that body temperature is not only influenced by the time of day, activity and individual fluctuations, but also depends on the measurement method. The type and location of the measurement have a (minor) effect on the measurement result:

Taking a fever in the anus (rectal): The measurement with the thermometer in the bottom is the most uncomfortable, but also the most accurate method. The measurement result obtained comes closest to the temperature inside the body.
Taking a fever under the tongue (sublingual): If the fever thermometer is placed under the tongue, good results are also obtained. These are usually 0.3 degrees lower than with the rectal measurement. When using the sublingual method, make sure that the lips are closed during the measurement (sometimes difficult with a blocked nose!). In addition, the patient must not have eaten or drunk anything cold or warm before the measurement, otherwise the result will be falsified.
Taking a fever in the ear (auricular): Taking a temperature in the ear using infrared waves is particularly useful for children, as it is quick and easy. To do this, the probe must be inserted into the ear canal, the easiest way to do this by gently pulling on the rear auricle. With otitis media and other ear diseases, it is better to measure the temperature in the healthy ear.
Taking a fever under the armpit (axillary): It is the most popular, but most inexact method of taking a fever.The measured temperature value can be up to 0.5 degrees below the actual temperature inside the body.

Fever: causes and possible diseases

Well known is the symptom of fever in the case of flu or a severe cold. In fact, infections with pathogens are the most common reason for an abnormally high body temperature. Sometimes, however, there are also non-infection-related diseases such as appendicitis, rheumatism or cancer behind it. So there are many possible causes of a fever. Here are some common examples:

Cold (flu-like infection) and flu
Pneumonia (caused by pathogens such as pneumococci)
Covid-19
Streptococcal infections such as purulent tonsillitis, blood poisoning (sepsis) or inflammation of the inner wall of the heart (endocarditis)
Pelvic inflammation
purulent abscesses
Appendicitis
tuberculosis
Inflammation of the blood vessels (vasculitis)
Connective tissue diseases (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 disease (Crohn's disease, ulcerative colitis)
chronic or alcohol-related inflammation of the liver (hepatitis)
Hormonal disorders (Addison's crisis, inflammation of the parathyroid gland, etc.)
Blockage of a blood vessel by a blood clot (thrombosis)
Allergies

Fever of Unknown Cause (FUO)

Doctors speak of a fever of unknown origin (FUO) when a patient's body temperature has been over 38.3 degrees Celsius for more than three weeks and no explanation can be found based on standard examinations.

In the case of many sick people, the cause of the FUO is ultimately found. Often these are previously undiscovered infections, tumor diseases, rheumatic diseases or autoimmune diseases. Medicines can also trigger a fever of unknown cause: For example, some people are hypersensitive to certain active ingredients in water tablets (diuretics), painkillers, antibiotics, sleeping pills or sedatives. The fever can be an expression of this "hypersensitivity".

A fever of unknown cause is also often observed in HIV patients. The trigger are often pathogens that normally - i.e. in people with a healthy immune system - do not cause an infection.

Causes of fever in children

Children are more likely to have a fever than adults. Usually even small infections are enough to raise the temperature. The guideline value from when a fever is present is defined in the same way as for adults. It is 38 degrees Celsius.

Fever in children is mostly caused by otitis media, gastrointestinal infections, bacterial respiratory infections with tonsillitis, cough, runny nose and sore throat. The little fever patients often suffer from a typical childhood disease such as scarlet fever, measles or three-day fever. In rare cases, a serious bacterial infection (such as pneumonia, meningitis) or a rheumatic disease is the reason for an abnormally high body temperature.

Fever: When do you need to see a doctor?

Fever rarely occurs alone, but is usually accompanied by other symptoms. Depending on the underlying illness, it can be headache and body aches, tiredness, chills, digestive problems such as diarrhea and vomiting, palpitations and / or clouding of consciousness.

When you go to the doctor, you should not only depend on your body temperature, but also on the general feeling of illness and the severity of the additional complaints. A high fever that has persisted for more than a day should definitely be treated by a doctor. The same applies if symptoms such as drowsiness, confusion or febrile seizures arise. Then call a doctor immediately!

Fever in children - when to see a doctor?

A pediatrician should be seen if:

the fever has lasted for more than a day
fever-lowering measures such as leg compresses or suppositories have no effect
additional symptoms such as barking cough, rash, diarrhea, vomiting or drowsiness
a febrile seizure occurs

For infants under three months of age, you should consult your pediatrician at a temperature of 38 degrees Celsius or more. If the baby does not develop a fever but shows the following symptoms, a doctor is also advisable:

The baby behaves differently than usual, is noticeably listless, tired, and reacts less.
Diarrhea, vomiting, or refusing to eat two meals a day are also reasons to see a doctor.
A rash or a change in skin color also indicates an illness and should be clarified by a doctor.

Emergency febrile seizure

An estimated four percent of all children suffer from febrile seizures at least once. Such an epilepsy-like attack can occur with infections and fever. It generally occurs between the ages of six months and five years. Small children between the ages of 14 and 18 months are most frequently affected.

A febrile seizure can be triggered by a rapid rise in temperature to 39 degrees Celsius and more. When it first appears, the signs are very frightening for many parents:

The child becomes unconscious for a short time, does not react and is no longer responsive.
The muscles of the whole body may stiffen or start to twitch.
The eyes are rolled or the child is staring.
Holding your breath makes your lips turn blue.

Most febrile seizures last three minutes or less and do not damage the brain. After the seizure, the affected children are usually drowsy and tired at first.

In most cases it is an uncomplicated febrile seizure: This is a febrile seizure that lasts less than 15 minutes, occurs in the typical age group (6 months to 5 years) and affects the whole brain (generalized). A complicated febrile seizure is less common: This is the case, for example, when a febrile seizure lasts longer than 15 minutes, recurs within 24 hours, occurs at an atypical age (5 years) or only affects a limited area of the brain (focal seizure).

Important: If a febrile seizure occurs for the first time or if it lasts longer than three minutes, you should inform the pediatrician or emergency doctor!

Fever: examinations

Since fever is a symptom, the underlying disease needs to be found. The doctor's first steps are:

Through the detailed questioning (anamnesis) of the patient or the parents (in the case of sick children), the doctor receives information on the possible causes of the fever. He asks, for example, how long the fever has been going on, whether there are any other complaints, whether the patient has recently had contact with sick people or animals, or has been abroad.
A blood test shows, among other things, whether there is currently an inflammation in the body (increased inflammation parameters such as CRP).
A physical exam can provide more information. For example, the doctor can listen to the patient's heart and lungs, measure blood pressure and pulse, palpate the abdomen and throat lymph nodes and take a look into the mouth, throat and ears.
Further examinations may be necessary if the previous findings are unclear or if there is a suspicion of a certain disease to be clarified. For example, the patient's sputum, urine and / or stool can be examined in the laboratory, the chest X-ray (chest X-ray), an EKG or an ultrasound of the heart, an ultrasound examination of the abdomen or special blood tests (for example for tuberculosis) .

Fever: treatment

What to do if you have a fever Fever is an important and natural defense reaction of the body against harmful influences. Viruses and bacteria can no longer multiply well at high temperatures. Therefore, fever does not have to be treated in every case. Basically, if you have a fever, be sure to keep bed rest! Simple remedies can also lower the fever.

When do you lower your fever?

When you should lower your fever depends on the cause, your physical condition, any previous illnesses and your personal level of suffering. If a child is severely affected by the fever and suffers, you should give the child a fever remedy from 38.5 degrees Celsius, but no later than 39 degrees.

Rules of conduct for a febrile seizure

If a child has a febrile seizure, parents should keep these tips in mind:

Look at the watch and measure how long the seizure has lasted.
Protect your child from injuries: Lay them down so that they cannot fall - for example in a cot or on the floor.
If the child is vomiting, you should turn them to the side so that the vomit can drain away and not be swallowed.
Loosen the child's clothes.
Do not put a bitestick or the like in the child's mouth in an effort to avoid biting the tongue - this often causes tooth damage worse than biting the tongue or cheek.
Do not try to stop the twitching.

There is no reliable prevention against febrile seizures. If your child is prone to febrile seizures, the most important thing to do is to prevent high temperatures. If the fever rises above 38.5 degrees Celsius, you should use home remedies for fever such as leg compresses or use fever suppositories. You should definitely discuss the dosage of the fever suppositories (or fever juice) with your pediatrician!

Home remedies for a fever

What to do if you have a fever In many cases, increased body temperature can be treated with simple antipyretic home remedies. These home remedies for fever have proven themselves over the decades:

Calf wrap if you have a fever

Leg wraps are a tried and true home remedy for fever. They dissipate excess heat from the body. Patients usually find the compresses very comfortable.

To do this, moisten thin linen or cotton towels with cool water. In adults, the temperature can be between 16 and 20 degrees Celsius. For calf wraps for babies, the water should be a little warmer (around 28 to 32 degrees Celsius). Wrap the towels tightly around the calves of the outstretched legs and secure with one or two layers of dry towels. The feet and the rest of the body should be kept warm.

After five minutes, take off the calf wrap again. But you can renew it two or three times. It is important to ensure that the fever is not lowered too quickly with the calf compresses - this would stress the circulation too much. In addition, do not do any calf compresses if you have chills!

You can find out more about the application in the article Leg wrap.

Quark wrap for a fever

Cold or body-warm quark pads or compresses also help with fever. To do this, spread 250 to 500 grams of cool or slightly warmed quark on a compress as thick as a finger and fold the fabric once. A protective layer of fabric should lie between the quark and the skin. Place around the calves and secure with gauze bandages or towels. Let it work for 20 to 40 minutes.

You can find out more about this home remedy in the article Quark compresses.

Abdominal and pulse wraps in the event of a fever

Another fever-lowering home remedy is the pulse wrap. To do this, dip cotton towels in cold water, wring them out and wrap them around the wrists and ankles. The wrap is particularly suitable for babies with a fever. A belly wrap can also help babies with a fever

You can read more about this in the article Wraps, pads and envelopes.

Bathing with a fever

You can also lower your fever with a cooling bath: The bathtub is first filled with warm water (the temperature should be around one degree Celsius below body temperature). Then you gradually add cold water to the foot of the tub until the bath water has sunk two to three degrees. Quit the bath after ten minutes. Afterwards, the patient should dry off well and go to bed.

Stop bathing immediately if the patient begins to chill or freeze.

A warm bath can also help to overcome a febrile infection. It promotes sweating and boosts the metabolism. This cold bath is helpful if you have a slight fever. The bath water is heated exactly to the patient's body temperature. Gradually add warm water to the foot of the tub until the temperature has risen to 39 to 40 degrees Celsius. Bathe for 20 minutes and then dry off well and rest well covered. Drink a lot while doing this.

If you have circulatory problems during the bath or if the temperature becomes uncomfortable, stop the bath immediately. The warming bath is not suitable for some heart diseases, neurological conditions and for small children.

You can read more about baths in the article hydrotherapy.

Drinks if you have a fever

Do you drink much! Patients with fever should ensure that they are drinking enough fluids. As a rule, from a body temperature of 37 degrees Celsius, an additional 0.5 to 1 liter of fluid is required for each increase of one degree (in addition to the normal amount of 1.5 to 2.5 liters consumed per day).

Water and unsweetened tea are suitable drinks. Linden blossom and elderflower teas are particularly recommended - they make you sweat and reduce fever. Meadowsweet tea can also lower a fever.

Fever medication

If the fever is high and the patient is weakened, antipyretic medication in the form of tablets, infusions, medicinal juices or suppositories can be useful. Effective ingredients are for example paracetamol, ibuprofen and acetylsalicylic acid. Discuss the use and dosage of such drugs with a pharmacist or doctor in advance.

Never give children with a fever the popular pain reliever and fever drug acetylsalicylic acid (ASA)! In connection with viral infections, it can trigger the life-threatening Reye's syndrome.

However, the most effective way to lower a fever is to treat the underlying condition. If, for example, a bacterial infection is behind the pathologically increased body temperature, the doctor can prescribe antibiotics.

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