A fever is an elevated body temperature. A temperature is considered elevated if it is higher than 38 ° C (with an oral thermometer) or higher than 38.2 °C (with a rectal thermometer). Many people take the term "fever" very broadly and often mean that they feel too warm, too cold or sweaty, but they have not actually taken their temperature.
Although 37 ° C is considered a normal temperature, body temperature varies throughout the day. It is lowest early in the morning and highest in the late afternoon and can then rise to as high as 37.7 ° C. Even a fever never stays at a constant temperature. Sometimes the temperature peaks daily and then returns to normal. This process is called intermittent fever. On the other hand, the temperature may also fluctuate but not drop to normal. This process is called remittent fever. Doctors no longer believe that the pattern of rising and falling fevers is very important in diagnosing certain diseases.
(See also Overview of infectious diseases Overview of infectious diseases Microorganisms are tiny living organisms such as bacteria and viruses. Microorganisms are present everywhere. Regardless of their overwhelming abundance, only a few of the thousands and thousands of species penetrate the. Learn more and fever in infants and children Fever in infants and children Normal body temperature varies from person to person and throughout the day (it is usually highest in the afternoon). In preschool-aged children, the normal. Learn more .)
Consequences of fever
The symptoms that people have are mainly triggered by the underlying disease and not by the fever itself.
Although many people are concerned that fever will harm them, a typical rise in body temperature to 38 to 40 °C, usually triggered by a short-lived (acute) infection, is well tolerated by most healthy adults. However, a moderate fever can be somewhat dangerous for adults with heart or lung disease because fever causes an increase in heart rate and respiratory rate. Fever can also worsen the mental condition of people with dementia.
Causes of fever in adults
Substances that produce fever are called pyrogens. Pyrogens can originate inside the body or come from the outside. Microorganisms and the substances they produce (such as toxins) are examples of pyrogens formed outside the body. Pyrogens that form inside the body are usually produced by monocytes and macrophages (two forms of white blood cells. Pyrogens that come from outside the body can cause fever by stimulating the body to release its own pyrogens or by directly affecting the area of the brain that regulates temperature Fever Natural barriers and the immune system protect the body from organisms that cause infections. (See also Lines of Defense.) Natural barriers include skin, mucous membrane. Learn more .
Infection is not the only cause of fever. Fever can also be the result of inflammation, a reaction to medication, an allergic reaction Overview of allergic reactions Allergic reactions (hypersensitivity reactions) are inappropriate reactions of the immune system to a normally harmless substance. Allergies usually lead to sneezing, watery eyes, and a lot of other symptoms. Learn more , an autoimmune disease Autoimmune disease is a malfunction of the immune system in which the body attacks its own tissues. What exactly leads to autoimmune disease is still unknown. Learn more (when the body produces antibodies that attack its own tissues) and an undetected cancer (especially leukemia Overview of Leukemias Leukemias are cancers of the white blood cells, or the cells that develop into white blood cells. White blood cells develop from the so-called stem cells. Learn more , Lymphoma Overview of Lymphoma Lymphomas are malignant (malignant) tumors of lymphocytes that occur in the lymphatic system and blood-forming organs. Lymphomas are cancers of a particular type of white blood cell. Learn More or kidney cancer Kidney cancer can cause blood in the urine, flank pain or fever. Cancer is usually discovered accidentally when an imaging procedure is used for another reason. The. Learn more ) its.
Many disorders can cause fever. They are broadly classified as:
Infectious (most common)
Adults with a fever that lasts 4 days or less are most likely to have an infection. If it is not an infection, the fever is more likely to last longer or return.
Many types of cancer cause fever.
Inflammatory diseases with fever affect the joints, connective tissues, and blood vessels, such as. B. Rheumatoid arthritis Rheumatoid arthritis (RA) is an inflammatory joint disease in which the joints, including most often the wrists and ankles, are inflamed. This results in swelling and pain, often followed by. Learn more , systemic lupus erythematosus Systemic lupus erythematosus (SLE) Systemic lupus erythematosus is a chronic inflammatory autoimmune disease of connective tissue that affects joints, kidneys, skin, mucous membranes, and the walls of blood vessels. Learn more (lupus) and giant cell arteritis Giant cell arteritis is a chronic inflammation of the large and medium-sized arteries in the head, neck, and upper body. Typically, the temporal lobe arteries are affected by. Learn more .
In addition, an isolated, short-lived (acute) fever in people with cancer or known inflammatory diseases most likely has an infectious cause. In healthy people, acute fever is very unlikely to be the first sign of chronic disease.
Most common causes
Almost all infectious diseases can cause fever. But overall, the following infectious causes are most likely:
Upper and lower respiratory infections
Most acute respiratory and gastrointestinal infections are caused by viruses.
Certain conditions (risk factors) make it more likely that people will develop a fever. These risk factors include:
Health status of the person
Age of the person
Use of certain medical procedures and medicines
Contact with infections (z. B. Through travel or contact with infected persons, animals, or insects)
Assessment of fever in adults
Usually, a doctor can determine if an infection is present with a brief history, a physical exam, and sometimes a few simple tests, such as a chest x-ray and a urine sample. However, sometimes the cause of a fever is not readily identifiable.
When physicians physically examine patients with acute fever, they focus on two basic questions:
If other symptoms such as headache or cough are present? These symptoms help narrow the range of possible causes.
Is there a severe or chronic illness? Many of the possible acute viral infections are difficult for doctors to diagnose (that is, it’s hard to determine which virus is causing the infection). Limiting testing to severely or chronically ill patients can help avoid many expensive, unnecessary, and often fruitless exams.
In people with acute fever, certain signs and characteristics are a cause for concern. These include
An impairment of mental functioning such as confusion
Headache, stiff neck, or both
Flat, small, red to purple spots on the skin (petechiae) that indicate bleeding under the skin
A racing heart or rapid breathing
A temperature above 40 °C or below 35 °C
Recent travel to an area where a serious infectious disease such as malaria is common (endemic)
Recent use of medications that suppress the immune system (immunosuppressants)
When to consult a doctor:
People with warning signs should see a doctor immediately. These usually require immediate examination and often hospitalization.
People without warning signs should call a doctor if the fever lasts more than 24 to 48 hours. Depending on the person’s age, other symptoms, and known medical conditions, the doctor may ask a patient to come to the doctor’s office for an examination or recommend treatment at home. Typically, sick people should see a doctor if the fever lasts more than 3 to 4 days, regardless of other symptoms.
What the doctor does:
Doctors always start by asking questions about the patient’s symptoms and medical history. They then perform a physical examination. What they find out from the medical history and physical exam often suggests the cause of the fever and useful testing.
The doctor first asks about current and past symptoms and illnesses, medications the patient is taking, blood transfusions, infection risks, recent travel, vaccinations, and recent hospitalizations, surgeries or other medical procedures. The pattern of the fever rarely helps the doctor make a diagnosis. Fever that returns every other or third day, but is typical of malaria. Doctors consider malaria as a possible cause only if people have been in a malaria area.
Recent travel can give physicians clues to the cause of a fever because some infections occur only in certain regions. For example, coccidioidomycosis (a fungal infection) occurs almost exclusively in the southwestern United States.
Also important is what recent risk one has been exposed to. For example, people who work in a meatpacking plant are more likely to have brucellosis (a bacterial infection spread through contact with pets). Other examples include contaminated water or risky foods (z. B. unpasteurized milk and dairy products, raw or semi-cooked meat, fish, and shellfish), insect bites and stings (such as from ticks or mosquitoes), unprotected sexual intercourse, and occupational or recreational risks (such as hunting, hiking, and water sports).
Pain is an important clue to the possible source of the fever. Therefore, doctors ask about pain in the ears, head, neck, teeth, throat, chest, abdomen, flanks, anus, muscles and joints.
Other symptoms that help determine the cause of the fever include stuffy and/or runny nose, cough, diarrhea, and urinary symptoms (frequency of micturition, urgency to urinate, and pain when urinating). Enlarged lymph nodes or rashes (including appearance, location and timing of appearance relative to other symptoms) can help the doctor pinpoint the cause. People with recurrent fever, night sweats and weight loss may be suffering from a chronic infection such as tuberculosis or endocarditis (an infection of the pericardium and usually the heart valve).
The doctor might also ask questions regarding:
Contact with an infected person
Known medical conditions that increase the risk of infection, such as AIDS, diabetes, cancer, an organ transplant, sickle cell anemia or heart valve disease, especially in patients with an artificial heart valve
Known diseases that increase the risk of infection such as lupus, gout, sarcoidosis, overactive thyroid (hyperthyroidism), or cancer
Taking medications that increase the risk of infection, such as cancer chemotherapy drugs, corticosteroids, or other drugs that suppress the immune system
Injection of drugs
Physical examination begins with confirmation of fever. Fever can be most accurately determined by taking the temperature rectally. The doctor then performs a thorough head-to-toe examination to look for a source of infection or evidence of disease.
The need for testing depends on the physician’s findings from the medical history and physical examination.
People who have an acute fever and only vague, general symptoms (who feel generally ill or have nonspecific complaints, for example) but are otherwise healthy probably have a viral illness that will heal without treatment. That is why they do not need tests. Exceptions are people who have been in contact with animals or insects (vectors) that carry and transmit a specific disease, such as people with a tick bite and people who have recently been in a region where certain diseases (such as malaria) are common.
If the findings in otherwise healthy people are suggestive of a particular disease, other tests may be needed. Doctors choose these tests based on the findings. In case of headache and stiff neck, z. B. A lumbar puncture is performed to diagnose meningitis (meningitis). If there is coughing and pulmonary congestion, a chest x-ray is taken to diagnose pneumonia.
Additional tests are often needed for patients at increased risk of infection, patients who appear severely ill, and elderly patients, even if a finding does not indicate a specific disease. In such patients, physicians often perform the following tests:
A complete blood count (including the number and percentage of different types of white blood cells)
A chest x-ray
An increase in the white blood cell count usually indicates an infection. The percentage of different types of white blood cells (differential blood count) provides further clues. For example, an increase in neutrophils suggests a relatively recent bacterial infection. An increase in eosinophils suggests a parasite infection such as tapeworms or nematodes. In addition, blood and other body fluids may be sent to a laboratory, where an attempt is made to grow the microorganisms in a culture. Other blood tests may be used to detect antibodies to certain microorganisms.
Fever of unknown cause
A fever of unknown cause can be diagnosed in the following cases:
If affected person has a fever of at least 38.3°C for several weeks
If a detailed examination does not reveal a cause
In such cases, an uncommon chronic infection may be the cause (such as. B. Tuberculosis Tuberculosis (TB) Tuberculosis is a contagious chronic infection caused by the bacterium Mycobacterium tuberculosis, which is transmitted by airborne infection. It usually involves the lungs. Learn more , a bacterial infection of the heart Infectious endocarditis Infectious endocarditis affects the inner lining of the heart (endocardium) and usually the heart valves as well. It occurs when bacteria enter the bloodstream. Learn more , an HIV infection Human immunodeficiency virus (HIV) infection is a viral infection that gradually destroys certain white blood cells and causes acquired immunodeficiency syndrome (AIDS). Learn more , the cytomegalovirus Cytomegalovirus infection (CMV infection) A cytomegalovirus infection is a common herpes virus infection with many types of symptoms, ranging from unnoticeable symptoms to fever and fatigue (the infectious mononucleosis. Learn more or the Epstein-Barr virus Infectious mononucleosis The Epstein-Barr virus causes a number of diseases, including infectious mononucleosis. The virus is spread by kissing. Symptoms vary, but most commonly. Learn More ) or something other than an infection, such as a connective tissue disease (like z. B. Lupus Systemic lupus erythematosus (SLE) Systemic lupus erythematosus is a chronic inflammatory autoimmune disease of connective tissue that affects joints, kidneys, skin, mucous membranes, and the walls of blood vessels. Learn more or rheumatoid arthritis Rheumatoid arthritis (RA) is an inflammatory joint disease in which the joints, including mostly the joints of the hands and feet, are inflamed. This results in swelling and pain, often followed by. Learn more ) or cancer (such as z. B. Lymphoma Overview of Lymphoma Lymphomas are malignant (malignant) tumors of lymphocytes that occur in the lymphatic system and blood-forming organs. Lymphomas are cancers of a particular type of white blood cell. Learn more or Leukemia Overview of Leukemias Leukemias are cancers of white blood cells, or the cells that develop into white blood cells. White blood cells develop from the so-called stem cells. Learn More ). Other causes include drug reactions, blood clots (deep vein thrombosis Deep vein thrombosis (DVT) In deep vein thrombosis, blood clots (thrombi) have formed in the deep veins, usually in the legs. Blood clots can form in a vein if it is injured. Learn more ), inflammation of organ tissue (sarcoidosis Sarcoidosis is a condition in which abnormal collections of inflammatory cells (granulomas) form in many organs in the body. Sarcoidosis usually occurs in 20- to 40-year-olds, most commonly affected. Learn more ) and inflammatory bowel disease Overview of inflammatory bowel disease (IBD) In inflammatory bowel disease, inflammation of the bowel occurs, often resulting in recurrent abdominal pain and diarrhea. The two most common chronic inflammatory. Learn more . In the elderly, the most common causes of a fever of unknown origin are: giant cell arteritis Giant cell arteritis is a chronic inflammation of the large and medium-sized arteries in the head, neck, and upper body. Typically, the temporal lobe arteries are affected by. Learn more , lymphoma, abscesses, and tuberculosis Tuberculosis (TB) Tuberculosis is a contagious chronic infection caused by the bacterium Mycobacterium tuberculosis, which is transmitted through the airways. It usually attacks the lungs. Learn more .
Doctors usually perform blood tests. These include a CBC, blood cultures, blood liver tests Blood liver tests are blood tests and are a noninvasive way to perform precautionary tests for liver disease (e.g., hepatitis). B. hepatitis in donated blood), and to test their. Learn more as well as tests to check for diseases of the connective tissue. Other tests, such as chest radiographs, urinalysis, and urine cultures, may be performed.
Ultrasound An ultrasound exam uses high-frequency sound waves (ultrasound) to create images of internal organs and other tissues. A device known as a "transducer" converts electrical. Learn More , Computed Tomography (CT) Computed tomography (CT) In a computed tomography (CT) scan, which was also called axial computed tomography (CAT) in the past, an X-ray source and X-ray detector rotate around a patient. The X-ray detector. Learn more and Magnetic resonance imaging (MRI) Magnetic resonance imaging (MRI) In a magnetic resonance imaging (MRI) very detailed images are produced with a strong magnetic field and high-frequency radio waves. For an MRI, no x-rays. Learn more , specifically in regions that cause discomfort, can help the doctor diagnose the cause. A radionuclide scan Radionuclide scanning involves the use of radionuclides to create images. A radionuclide is the radioactive form of an element, i.e., an unstable atom, that is produced by the release of. Learn more can be performed after injecting radiolabeled white blood cells into the vein to identify regions of infection or inflammation.
If the test results are negative, the doctor may need to take a tissue sample from the liver, bone marrow, or other site where infection is suspected for biopsy. The sample is then examined under a microscope, cultured and analyzed.
Treatment of fever with unknown cause focuses on treating the condition causing the fever, if known. Doctors sometimes prescribe medicines to lower body temperature (see Treating a fever Fever is an elevated body temperature. A temperature is considered elevated if it is higher than 38°C (with an oral thermometer) or higher than 38.2°C (with a rectal thermometer). Learn More ).
Treating fever in adults
Because fever helps the body fight off infection, and because fever itself is not dangerous (unless it is higher than about 41.1°C), there is debate about whether fever should be treated routinely. However, people with high fevers generally feel much better when their fever is treated. In addition, patients who have heart or lung disease or dementia are considered especially vulnerable to dangerous complications. This is why they should be treated when they have a fever.
Drugs that lower body temperature are called antipyretics.
The most effective and common antipyretics are acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, and naproxen.
Patients usually take one of the following:
650 milligrams of acetaminophen every 6 hours (no more than 4000 milligrams per day)
200 to 400 milligrams of ibuprofen every 6 hours
Because many over-the-counter cold or flu medications contain acetaminophen, people must be careful not to take multiple doses of this drug.
Other cooling measures (such as cooling with lukewarm water mist and cooling blankets) are needed only if the temperature rises above 41.1 °C. Rubbing with alcohol should be avoided, as alcohol is absorbed by the skin and can have harmful effects.
People with a blood infection or abnormal vital signs (such as low blood pressure, rapid pulse rate, and rapid breathing rate) are admitted to the hospital.
Important information for older people: Fever
Fever in older people can be difficult because the body may not respond as it does in younger people. For example, frail, elderly people are less likely to have an infection causing the fever. Even when the temperature is elevated by infection, it may be lower than the standard definition of fever, and the level of fever may not be consistent with the severity of the illness. Likewise, other symptoms such as pain might be less noticeable. A change in mental functioning or a decrease in the ability to perform daily tasks is often the only other indication of pneumonia nosocomial acquired pneumonia Nosocomial acquired pneumonia is pneumonia that occurs in hospitalized inpatients, usually after at least two days of hospitalization. Learn more or a urinary tract infection Overview of urinary tract infections (UTIs) In healthy people, urine in the bladder is sterile; it contains neither bacteria nor other pathogens. Also in the urethra, the channel through which urine from the bladder leaves the body. Learn more .
However, older people with fever are more likely to have a severe bacterial infection than younger adults with fever. As in younger adults, the cause is usually a respiratory or urinary tract infection. Skin and soft tissue infections are also common causes in the elderly. Older people are more likely to have severe manifestations of respiratory viral infections such as z. B. Influenza Influenza (flu) is a viral infection of the lungs and respiratory tract with one of the influenza viruses. It causes fever, cold, sore throat, cough, headache and muscle pain (myalgia). Learn more and COVID-19 COVID-19 coronaviruses are a large family of viruses that cause respiratory illnesses ranging from the common cold to deadly pneumonia. There are many. Learn more .
Diagnosis of a fever in the elderly is similar to that in younger adults, but physicians usually recommend urine tests (with culture) and a chest x-ray in the elderly. Blood samples are cultured to detect a blood infection (bacteremia Bacteremia A bacteremia means that there are bacteria in the bloodstream. Bacteremia can have simple causes (such as vigorous tooth brushing) or from dental or medical procedures. Learn more ).
Main points about fever in adults
In healthy people, fever is usually triggered by a viral respiratory or gastrointestinal infection.
Doctors can usually identify an infection from a brief history and physical examination, and sometimes from a few simple tests. Doctors use the results, and especially the symptoms, to determine what additional tests are needed.
Doctors consider underlying chronic diseases, especially those that compromise the immune system, as a possible cause of prolonged fever.
Taking acetaminophen or an NSAID usually reduces the fever and helps people feel better, although in most people treatment is not essential.
In the elderly, infections are less likely to cause fever and other symptoms may be less noticeable.