Fever is when the body temperature is above 38ºC. It is a natural defense mechanism of the body, which hinders the development of most microorganisms, helping the immune system to fight infections. For this reason, fever is a very non-specific symptom, as it can have different causes, from flu, tonsillitis, COVID-19, autoimmune diseases and even cancer, for example.
Although the value of 38ºC is used to define fever, the truth is that fever is any elevation in temperature above the normal range for each person. This is why some people may feel “feverish” and generally unwell even when the temperature is still close to 37.5ºC.
Whenever the fever is very high (above 40ºC) or persists for more than 3 days, it is important to consult a general practitioner to identify the cause and begin the most appropriate treatment.
How to know if it’s a fever
If you think you may have a fever, please enter your temperature value into the calculator:
Understand better what temperature is a fever and when to worry.
Main causes of fever
The main causes of fever are:
1. Infections
Infections with viruses, bacteria or parasites are the main cause of fever. The most common include:
- Pneumonia;
- Gripe;
- Sinusitis;
- COVID-19;
- Malaria;
- Because.
In addition to fever, it is common for other symptoms to appear, including respiratory changes, headache, malaise, excessive tiredness or weakness, depending on the type of infection.
What to do: It is important to consult a general practitioner to carry out tests that help identify the infectious agent and, thus, initiate the most appropriate treatment, which may involve the use of antibiotics or antivirals. In addition, rest may be recommended.
2. Autoimmune diseases
Some autoimmune diseases, such as rheumatoid arthritis and Crohn’s disease, can also cause a high, persistent fever. This happens because the body’s healthy cells are being attacked, leading to the development of chronic inflammation, which results in an increase in body temperature.
In the case of rheumatoid arthritis, in addition to fever, it is also common to have pain and stiffness in the joints, redness and difficulty moving the affected joint. In the case of Crohn’s disease, symptoms other than fever are intense diarrhea, abdominal cramps and weight loss, for example.
What to do: It is important to follow the doctor’s instructions, which normally indicate the use of medications that reduce the activity of the immune system, promoting symptom control. Additionally, the use of anti-inflammatory medications or corticosteroids may be recommended to alleviate symptoms. In the case of Crohn’s disease, it is essential that the person also has the guidance of a nutritionist, to follow a diet that prevents attacks. See what food should be like for Crohn’s disease.
3. Vaccines
Some vaccines may cause fever as an adverse reaction. This happens mainly in vaccines that are manufactured with an attenuated form of the infectious agent or toxin, which causes the immune system to produce antibodies that, in addition to stimulating immunity, can also produce fever.
Some of the vaccines that have fever as a side effect are: the tetanus vaccine, the diphtheria vaccine and the COVID-19 vaccines, for example.
What to do: fever is considered normal after vaccination, disappearing after a few hours without the need for any type of treatment. However, to alleviate the fever, you can take a warm bath, stay in a ventilated environment and place a towel damp with cold water on your forehead and wrists, for example. See more ways to lower your fever.
4. Use of medications
Some medications such as antimicrobials, antihistamines, antidepressants, opioids, antiemetics and central nervous system stimulants, for example, can also have fever as a side effect.
What to do: If you suspect that the fever is caused by the use of any medicine, it is important to inform the doctor, in order to assess the need to change or suspend the use of the medicine.
5. Cancer
Fever may be present in some types of cancer, especially in the more advanced stages, in which other organs are affected by tumor cells. Check out the symptoms that may indicate cancer.
What to do: It is important to inform your doctor of the symptoms you are experiencing and follow the recommended treatment. In some cases, it may be necessary to adjust cancer treatment to prevent further progression of the disease.
How to measure body temperature
To find out body temperature and confirm fever, it is recommended to use a thermometer: digital or glass (which must be placed in the armpit, mouth or anus) or infrared (which simply needs to be pointed at the forehead).
See how to measure temperature correctly.
