Fever is an elevated body temperature that occurs when the body's thermostat (located in the hypothalamus in the brain) resets at a higher temperature, primarily in response to an infection. Elevated body temperature that is not caused by a resetting of the temperature set point is called hyperthermia.
Although 98.6° F (37° C) is considered normal temperature, body temperature varies throughout the day. It is lowest in the early morning and highest in the late afternoon—sometimes reaching 99.9° F (37.7° C). Similarly, a fever does not stay at a constant temperature. Sometimes temperature peaks every day and then returns to normal. This process is called intermittent fever. Alternatively, temperature varies but does not return to normal. This process is called remittent fever. Doctors no longer think that the pattern of the rise and fall of fever is very important in the diagnosis of certain disorders.
Body temperature can be measured at several areas of the body. The most common sites are the mouth (oral) and rectum. Other sites include the ear, forehead, and, much less preferably, armpit. Temperature is typically measured using a digital thermometer. Glass thermometers containing mercury are no longer recommended because they can break and expose people to mercury.
Oral temperatures are considered elevated when
They are higher than 99° F (37° C) in the early morning.
They are higher than 100.4° F (38° C) at any time after the early morning.
They are higher than a person’s known normal everyday temperature.
Rectal and ear temperatures are about 1.0° F (0.6° C) higher than oral temperatures.
Skin temperatures (for example, the forehead) are about 1.0° F (0.6° C) lower than oral temperatures.
Many people use the term “fever” loosely, often meaning that they feel too warm, too cold, or sweaty, but they have not actually measured their temperature.
The ability to generate a fever is reduced in certain people (for example, those who are very old, very young, or who have an alcohol use disorder).
(See also Overview of Infectious Disease and Fever in Infants and Children.)
Consequences of fever
The symptoms people have are due mainly to the condition causing the fever rather than the fever itself.
Although many people worry that fever can cause harm, the typical temporary elevations in body temperature ranging from 100.4° to 104° F (38° to 40° C) caused by most short-lived (acute) infections are well-tolerated by healthy adults. However, a moderate fever may be slightly dangerous for adults with a heart or lung disorder because fever causes the heart rate and breathing rate to increase. Fever can also worsen mental status in people with dementia.
Extreme temperature elevation (typically more than 105.8° F, or 41° C) may be damaging. A body temperature this high can cause malfunction and ultimately failure of most organs. Such extreme elevation sometimes results from very severe infection (such as sepsis, malaria, or meningitis) but is more typically caused by heatstroke or use of certain drugs.
Fever in healthy children can cause febrile seizures.
Causes of Fever in Adults
Substances that cause fever are called pyrogens. Pyrogens can come from inside or outside the body. Microorganisms and the substances they produce (such as toxins) are examples of pyrogens formed outside the body. Pyrogens formed inside the body are usually produced by monocytes and macrophages (two types of white blood cells). Pyrogens 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 controls temperature.
Infection is not the only cause of fever. Fever may also result from inflammation, a reaction to a drug, an allergic reaction, autoimmune disorders (when the body produces abnormal antibodies that attack its own tissues), and undetected cancer (especially leukemia, lymphoma, or kidney cancer).
Many disorders can cause fever. They are broadly categorized as
Infectious (most common)
Neoplastic (cancer)
Inflammatory
An infectious cause is highly likely in adults with a fever that lasts 4 days or less. A noninfectious cause is more likely to cause fever that lasts a long time or returns.
Many cancers cause fever.
Inflammatory disorders that cause fever include joint, connective tissue, and blood vessel disorders such as rheumatoid arthritis, systemic lupus erythematosus (lupus), and giant cell arteritis.
Also, an isolated, short-lived (acute) fever in people with cancer or a known inflammatory disorder is most likely to have an infectious cause. In healthy people, an acute fever is unlikely to be the first sign of a chronic illness.
Drugs sometimes cause fever. For example, beta-lactam antibiotics (such as penicillin) and sulfa drugs can trigger a fever. Drugs that can cause an extremely high temperature include certain illicit drugs (such as , amphetamines, or phencyclidine), anesthetics, and antipsychotic drugs.
Most common causes
Virtually all infectious disorders can cause fever. But overall, the most likely infectious causes are
Upper and lower respiratory tract infections
Gastrointestinal infections
Urinary tract infections (UTIs)
Most acute respiratory tract and gastrointestinal infections are viral.
Risk factors
Certain conditions (risk factors) make people more likely to have a fever. These factors include the following:
The person's health status
The person's age
Certain occupations
Use of certain medical procedures and drugs
Exposure to infections (for example, through travel or contact with infected people, animals, or insects)
Evaluation of Fever in Adults
Usually, a doctor can determine that an infection is present based on a brief history, a physical examination, and occasionally a few simple tests, such as a chest x-ray and urine tests. However, sometimes the cause of fever is not readily identified.
When doctors initially evaluate people with an acute fever, they focus on two general issues:
Identifying other symptoms such as headache or cough: These symptoms help narrow the range of possible causes.
Determining whether the person is seriously or chronically ill: Many of the possible acute viral infections go away on their own and also are difficult for doctors to diagnose specifically (that is, to determine exactly which virus is causing the infection). Limiting testing to people who are seriously or chronically ill can help avoid many expensive, unnecessary, and often fruitless searches.
Warning signs
In people with an acute fever, certain signs and characteristics are cause for concern. They include
A change in mental function, such as confusion
A headache, stiff neck, or both
Flat, small, purplish red spots on the skin (petechiae), which indicate bleeding under the skin
Low blood pressure
Rapid heart rate or rapid breathing
Shortness of breath (dyspnea)
A temperature that is higher than 104° F (40° C) or lower than 95° F (35° C)
Recent travel to an area where a serious infectious disease such as malaria is common (endemic)
Recent use of drugs that suppress the immune system (immunosuppressants)
When to see a doctor
People who have any warning signs should see a doctor right away. Such people typically need immediate testing and often admission to a hospital.
People without warning signs should call the 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 the person to come for evaluation or recommend treatment at home. Typically, people should see a doctor if a fever lasts more than 3 or 4 days regardless of other symptoms.
What the doctor does
Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause of the fever and the tests that may need to be done.
A doctor begins by asking a person about present and previous symptoms and disorders, drugs currently being taken, any blood transfusions, exposure to infections, recent travel, vaccinations, and recent hospitalizations, surgeries, or other medical procedures. The pattern of the fever rarely helps the doctor make a diagnosis. However, a fever that returns every other day or every third day is typical of malaria. Doctors consider malaria as a possible cause only if people have traveled to an area where malaria is common.
Recent travel may give the doctor clues to the cause of a fever because some infections occur only in certain areas. For example, coccidioidomycosis (a fungal infection) occurs almost exclusively in the southwestern United States.
Recent exposures are also important. For example, people who work in a meatpacking plant are more likely to develop brucellosis (a bacterial infection spread through contact with domestic animals). Other examples include unsafe water or food (such as unpasteurized milk and milk products, and raw or undercooked meat, fish, and shellfish), insect (such as mosquitoes) and tick bites, unprotected sex, and occupational or recreational exposures (such as hunting, hiking, and water sports).
Pain is an important clue to the possible source of fever, so the doctor asks about any pain in the ears, head, neck, teeth, throat, chest, abdomen, flank, rectum, muscles, and joints.
Other symptoms that help determine the cause of the fever include nasal congestion and/or discharge, cough, diarrhea, and urinary symptoms (frequency, urgency, and pain while urinating). Knowing whether the person has enlarged lymph nodes or a rash (including what it looks like, where it is, and when it appeared in relation to other symptoms) may help the doctor pinpoint a cause. People with recurring fevers, night sweats, and weight loss may have a chronic infection such as tuberculosis or endocarditis (infection of the heart's lining and usually the heart valves).
The doctor may also ask about the following:
Contact with anyone who has an infection
Any known conditions that predispose to infection, such as HIV infection, diabetes, cancer, organ transplantation, sickle cell disease, pacemaker, or heart valve disorders, particularly if an artificial valve is present
Any known disorders that predispose to fever without infection, such as lupus, gout, sarcoidosis, an overactive thyroid gland (hyperthyroidism), or cancer
Use of any drugs that predispose to infection, such as cancer chemotherapy drugs, corticosteroids, or other drugs that suppress the immune system
Use of illicit drugs that are injected
The physical examination begins with confirmation of fever. Fever is most accurately determined by measuring rectal temperature, but oral or ear temperature is often measured. Forehead temperature is not as accurate as rectal, but because of the COVID-19 pandemic, doctors may measure forehead temperature to screen people for fever. Armpit temperature is the least accurate, so doctors rarely use it to confirm a fever. Doctors then do a thorough examination from head to toe to check for a source of infection or evidence of disease.
Testing
The need for testing depends on what the doctor finds during the medical history and physical examination.
Otherwise healthy people who have an acute fever and only vague, general symptoms (for example, they feel generally ill or achy) probably have a viral illness that will go away without treatment. Therefore, they do not usually require testing. Exceptions are people in an area where COVID-19 is being spread, people who have been exposed to an animal or insect that carries and transmits a specific disease (called a vector), such as people with a tick bite, and people who have recently been in an area where a particular disorder (such as malaria) is common.
If otherwise healthy people have findings that suggest a particular disorder, testing may be needed. Doctors select tests based on those findings. For example, if people have a headache and stiff neck, a spinal tap (lumbar puncture) is done to look for meningitis. If people have a cough and lung congestion, a chest x-ray is done to look for pneumonia. For certain respiratory diseases, such as COVID-19 or influenza, there are rapid molecular tests that can give results within minutes or a few hours. Some rapid tests can be done at home or in a doctor's office on a sample obtained with a cotton swab. Other rapid tests to identify a specific cause of an infection require a blood sample sent to a laboratory.
People who are at increased risk of infection, people who appear seriously ill, and older people often need testing even when findings do not suggest a particular disorder. For such people, doctors may do the following:
A complete blood count (including the number and proportion of different types of white blood cells)
Urine and blood cultures
A chest x-ray
Urinalysis
An increase in the white blood cell count usually indicates infection. The proportion of different types of white blood cells (differential count) gives further clues. For example, an increase in neutrophils suggests a relatively new bacterial infection. An increase in eosinophils suggests the presence of parasites, such as tapeworms or roundworms. Also, blood, urine, and other body fluids may be sent to the laboratory to try to grow the microorganism in a culture. Other blood tests can be used to detect antibodies against specific microorganisms.
Fever of unknown origin (FUO)
A fever of unknown origin may be diagnosed when
People have a fever of at least 101° F (38.3° C) for several weeks.
Extensive investigation does not detect a cause.
In such cases, the cause may be an unusual chronic infection (such as tuberculosis, bacterial infection of the heart, HIV infection, cytomegalovirus, or Epstein-Barr virus) or something other than an infection, such as a connective tissue disorder (such as lupus or rheumatoid arthritis) or cancer (such as lymphoma, ovarian cancer, or leukemia). Other causes include drug reactions, blood clots (deep vein thrombosis), inflammation of organ tissues (sarcoidosis), and inflammatory bowel disease. In older people, the most common causes of FUO are giant cell arteritis, lymphomas, abscesses, and tuberculosis.
Doctors usually do blood tests, including a complete blood count, blood cultures, liver blood tests, and tests to check for connective tissue disorders. Other tests, such as chest x-ray, urinalysis, and urine culture, may be done.
Ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI), particularly of areas that are causing discomfort, may help a doctor diagnose the cause. Radionuclide scanning, done after white blood cells labeled with a radioactive marker are injected into a vein, or positron emission tomography (PET) may be used to identify areas of infection or inflammation.
If these test results are negative, doctors may need to take a sample of tissue from the liver, bone marrow, or another site of suspected infection for biopsy. The sample is then examined under a microscope, cultured, and analyzed.
The treatment of FUO is focused on treating the disorder causing the fever if it is known. Doctors may give drugs to lower the body temperature (see treatment of fever).
Treatment of Fever in Adults
The best fever treatment is treatment of the cause.
Because fever helps the body defend against infection and because fever itself is not dangerous (unless it is higher than about 106° F [41° C]), there is some debate as to whether fever should be routinely treated. However, people with a high fever generally feel much better when the fever is treated. Plus, people with a heart or lung disorder and those with dementia are considered to be at particular risk of dangerous complications, so when they have a fever, it should be treated.
Drugs used to lower body temperature are called antipyretics.
Because many over-the-counter cold or flu preparations contain acetaminophen, people must be careful not to take acetaminophen and one or more of these preparations at the same time.
Other cooling measures (such as cooling with a tepid water mist and using cooling blankets) are needed only if the temperature is about 106° F (41.1° C) or higher. Sponging with alcohol is avoided because alcohol can be absorbed through the skin and may have harmful effects.
People who have a blood infection or who have abnormal vital signs (such as low blood pressure and a rapid pulse and breathing rate) are admitted to the hospital.
Essentials for Older People: Fever
Fever can be tricky in older people because the body may not respond the way it would in younger people. For example, in frail older people, infection is less likely to cause fever. Even when elevated by infection, the temperature may be lower than the standard definition of fever, and the degree of fever may not correspond to the severity of the illness. Similarly, other symptoms, such as pain, may be less noticeable. Frequently, a change in mental function or a decline in daily functioning is the only other initial sign of pneumonia or a urinary tract infection.
However, older people with a fever are more likely to have a serious bacterial infection than are younger adults with a fever. As in younger adults, the cause is commonly a respiratory or urinary tract infection. Skin and soft-tissue infections are also common causes in older people. Older people are more likely to have serious manifestations of respiratory viral infections such as influenza, COVID-19, and respiratory syncytial virus (RSV).
Diagnosis of fever in older people is similar to that for younger adults, except that for older people, doctors usually recommend urine tests (including culture) and a chest x-ray. Samples of blood are cultured to rule out a blood infection (bacteremia).
Key Points
Most fevers in healthy people are caused by a respiratory or gastrointestinal infection due to a virus.
If people with a fever have any warning signs, they should see a doctor right away.
Doctors can usually identify an infection based on a brief medical history, a physical examination, and occasionally a few simple tests, and then doctors use these results, particularly symptoms, to determine which other tests are needed.
Doctors consider underlying chronic disorders, particularly those that impair the immune system, as a possible cause of fever that lasts a long time.
In older people, infections are less likely to cause fever, and other symptoms may be less noticeable.
