How does a fever develop?
Fever is caused by a complex reaction of the body, which is usually triggered by the immune system when it recognizes pathogens or other disorders.
Fever is triggered by so-called pyrogens (fever-inducing substances). These can be divided into two groups:
- Exogenous pyrogens: These enter the body from outside, e.g. components of bacteria (such as lipopolysaccharides) or toxins from viruses.
- Endogenous pyrogens: These are produced by the body itself, e.g. certain cytokines (interleukin-1, interleukin-6, tumor necrosis factor alpha) in response to the presence of pathogens or other triggers (e.g. tumor cells).
These pyrogens activate immune cells, which then send signals to the hypothalamus, the body’s temperature regulation center. Based on the signals received, the hypothalamus increases the set point for normal body temperature. This is normally around 37 °C. In the event of a fever, for example, the setpoint is increased to 39 °C. The body then takes measures to increase the temperature.
These include:
- Muscle tremors (shivering): The muscles contract repeatedly to generate heat.
- Vascular constriction (vasoconstriction): The blood vessels in the skin contract in order to reduce heat loss.
- Increased metabolism: The body uses more energy to produce heat.
Once the cause of the fever, e.g. an infection, has been dealt with, the body stops producing pyrogens. The set point in the hypothalamus drops back to the normal temperature. The body reacts with sweating and vasodilation to release excess heat and lower the temperature.
Causes of fever
Fever can have a variety of causes. Here are the main reasons why people develop a fever.
Combating infections
Increased body temperature can inhibit the multiplication of certain pathogens, as fever creates an environment that is unfavorable for many bacteria and viruses. Fever can also increase the activity of certain immune cells such as white blood cells, which help to fight pathogens.
Inflammatory reaction
Fever often occurs with inflammation. The inflammation may be the result of an infection or of a non-infectious nature. The body releases so-called pyrogens (fever-inducing substances), which signal the hypothalamus to increase the body temperature.
Reaction to injuries or illnesses
In addition to infections, fever can also be triggered by other conditions, e.g. autoimmune diseases, cancer (hyperthermia) or side effects of medication. The body tries to react to these stress situations by increasing its temperature. A distinction is made between fever and hyperthermia, such as heat stroke. In contrast to fever, however, the temperature set point in the hypothalamus is not increased.
Promotion of healing processes
A higher temperature accelerates certain biochemical processes in the body that can contribute to healing, such as tissue repair.
External influences
A number of external influences can lead to fever:
- Toxins: Contact with toxic substances or contaminants (e.g. food poisoning caused by bacteria such as salmonella) can cause febrile reactions.
- Severe allergic reactions, especially to medication or insect bites, can sometimes trigger a fever.
- Some medications or substances can cause fever as a side effect. This reaction is also known as drug-induced fever.
When is a fever dangerous?
In principle, fever is a normal defense reaction of the body and in many cases not dangerous. It indicates that the immune system is actively working against pathogens. However, a fever can become dangerous if it rises too high or lasts too long. In adults, a temperature above 39.5 °C is considered a high fever. In children, even lower temperatures can be critical, especially if they are accompanied by febrile convulsions. People with cardiovascular diseases can also suffer complications due to the increased stress caused by fever.
The level of fever alone does not always provide information about the severity of an illness. Some mild infections can be accompanied by a high fever, while severe illnesses sometimes only cause a slight increase in temperature. It is therefore important to be aware of the accompanying symptoms of fever. A fever requires rapid medical clarification if it is associated with the following symptoms:
- Impairment of mental functioning (confusion, lethargy)
- Severe headache, stiff neck or both
- Small, red to purple spots on the skin (petechiae) that cannot be pressed away, indicating bleeding under the skin
- Low blood pressure
- Palpitations, rapid breathing or shortness of breath
- A temperature above 40 °C or below 35 °C
People who have recently been to areas where serious infectious diseases (malaria, dengue fever) are widespread or who are taking medication that suppresses the immune system should also seek medical treatment quickly if they have a fever.
If none of the above-mentioned accompanying symptoms occur and the general condition is not significantly impaired, medical advice should be sought if the fever persists for more than 3 to 4 days.
Should you reduce fever and how?
Fever does not always need to be brought down immediately, as it is part of the body’s natural defenses. Doctors recommend taking antipyretic measures from a temperature of around 39 °C, unless those affected find the fever very distressing. For children, older people or people with pre-existing conditions such as heart problems, it may be advisable to intervene at lower temperatures.
Both medication and home remedies are available to reduce fever:
Antipyretic medication
- Paracetamol and ibuprofen are proven antipyretics. They also alleviate accompanying symptoms such as headaches and aching limbs.
Home remedies for fever
- Calf compresses: These are a tried and tested household remedy for gently lowering body temperature. A liter of lukewarm water with two tablespoons of lemon juice or vinegar serves as a base. Wet two towels or kitchen towels in it, wring them out and place them damp on each calf. Wrap a dry towel around each leg and rest in bed for around 15 minutes. If the temperature does not drop, repeat. Important: Only use when the legs and feet are warm. If your feet are cold, you should refrain from doing so.
- Drink plenty of fluids: Fever leads to fluid loss due to increased sweating, so it is important to drink plenty of fluids. In addition to water, bouillon, tea and diluted juices, there are also drinks that can have a fever-reducing effect: Two tablespoons of apple cider vinegar with a little honey in a glass of water or ginger tea made from fresh ginger can help the body to reduce fever. Lime blossom tea is also considered to be diaphoretic and antipyretic.
- Light clothing: Wear light clothing to avoid overheating your body. If you suffer from chills, a blanket and a bed bottle can of course help.
- Rest and sleep: Stay at home if you have a fever, preferably in bed. Avoid strenuous activities and get plenty of rest. Sleep is particularly important so that your body can recover.
Conclusion: Fever is an important mechanism used by the body to fight infections. In most cases it is harmless and does not require immediate treatment. Nevertheless, you should be careful, especially if you have a high or prolonged fever or corresponding accompanying symptoms.
Frequently asked questions about fever
If the fever does not go down or even worsens despite fever-reducing measures, a doctor should be consulted immediately. This is particularly true if other symptoms such as shortness of breath, severe pain or confusion are also present. It could be a serious infection that requires specific treatment.
If you have a fever, you should stay at home and rest until your temperature returns to normal and you feel fit. It is generally recommended to be fever-free for at least 24 hours before going back to work or school. This not only helps with your own recovery, but also reduces the risk of infecting others.
No, physical exertion can put additional strain on the body and promote complications such as myocarditis.
After a feverish illness, the body should be given sufficient time to recover. Sport is only advisable again once you have been free of fever and symptoms for at least two days. Returning to sporting activities too early can put a strain on the heart and, in the worst case, lead to myocarditis.
Chills and shivering during a fever are a consequence of the body’s temperature regulation. The hypothalamus in the brain increases the body temperature set point in order to fight pathogens more effectively. As long as the body temperature is still below this new set point, you freeze and shiver in order to increase your body temperature. As soon as the setpoint is reached, the freezing subsides
Not mandatory. The height of the fever does not always provide information about the severity of an illness. Some mild infections can be accompanied by a high fever, while severe illnesses sometimes only cause a slight increase in temperature.
The most accurate method of temperature measurement is in the ear (tympanic temperature measurement). Ear measurement is also the gold standard for children from around six months of age, provided the middle ear is healthy and the ear canal is free of blockages or inflammation. Alternatively, the temperature can also be measured under the armpit, but this method is less precise and usually shows values that are around 0.5 °C lower than the temperature measured inside the body.
Fever should usually be measured two to three times a day in order to monitor the course of the temperature. The measurement is particularly important after taking antipyretic medication. However, only after at least 30 to 60 minutes after taking the medication in order to obtain a realistic assessment of the effect.
If the fever does not go down or even worsens despite fever-reducing measures, a doctor should be consulted immediately. This is particularly true if other symptoms such as shortness of breath, severe pain or confusion are also present. It could be a serious infection that requires specific treatment.
A fever is when the body temperature exceeds certain values. A single measurement above 38.3 °C is considered a fever. Alternatively, a temperature above 38.0 °C that persists for at least one hour is also considered a fever.
Other values apply for children and infants. Find out more about fever on the Kispi website.
It is important to note that these temperature values are guidelines and individual deviations are normal. Adults should learn to know the normal temperature ranges for themselves and their family members so that they can better assess when medical help is needed in the event of illness
Fever itself does not, but often the underlying infection, e.g. viruses or bacteria.
More important than special foods are sufficient fluids and a light, nutritious diet. Water, herbal teas or broths are good for keeping the body hydrated.
