Fever is an elevated body temperature. Temperature is considered elevated when it is higher than 100° F (37.8° C) as measured by an oral thermometer or higher than 100.8° F (38.2° C) as measured by a rectal thermometer. 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.
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—a process called intermittent fever. Alternatively, temperature varies but does not return to normal—a process called remittent fever. Doctors no longer think that the pattern of the rise and fall of fever is very important in diagnosis of certain disorders.
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 to 100.4° to 104° F (38° to 40° C) caused by most short-lived (acute) illnesses 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 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 (see Heat Disorders: Heatstroke) or use of certain drugs. Drugs that can cause an extremely high temperature include certain illicit drugs (such as cocaine, amphetamines, or phencyclidine), anesthetics, and antipsychotic drugs.
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 cause fever by stimulating the body to release its own pyrogens. However, 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 or lymphoma).
Many disorders can cause fever. They are broadly categorized as
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 recurs. Many cancers and inflammatory disorders cause fever. Inflammatory disorders include joint, connective tissue, and blood vessel disorders such as rheumatoid arthritis, systemic lupus erythematosus (lupus), and giant cell arteritis.
Drugs sometimes cause fever.
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.
Most common causes:
Virtually all infectious disorders can cause fever. But overall, the most likely causes are
Most acute respiratory tract and gastrointestinal infections are viral.
Certain conditions (risk factors) make people more likely to have a fever. These factors include the person's health status, age, certain occupations, and use of certain medical procedures and drugs, as well as exposure to infections (for example, through travel or contact with infected people or insects).
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Some Causes of Fever
Upper or lower respiratory tract infection
Urinary tract infection
Infection related to a catheter inserted in a vein (IV catheter infection)
Urinary tract infection, particularly in people with a urinary catheter
Pneumonia, particularly in people on a ventilator
Atelectasis (collapse of part of a lung due to an airway blockage, rather than an infection)
Infection or a pocket of blood (hematoma) at the site of surgery
Deep vein thrombosis or pulmonary embolism
Diarrhea (due to Clostridium difficile–induced colitis)
Travel to areas where an infection is common (endemic areas)
Disorders that cause diarrhea
Dengue fever (less common)
Exposure to insects or animals that carry disease-causing organisms, called vectors (in the United States)
Ticks: Rickettsiosis, ehrlichiosis, anaplasmosis, Lyme disease, babesiosis, or tularemia
Mosquitoes: Arboviral encephalitis
Wild animals: Tularemia, rabies, or hantavirus infection
Domestic animals: Brucellosis, cat-scratch disease, Q fever, or toxoplasmosis
Reptiles: Salmonella infection
Bats: Rabies or histoplasmosis
A weakened immune system (immunocompromise)
Viruses: Varicella-zoster virus or cytomegalovirus infection
Bacteria: Infection due to pneumococci, meningococci, Staphylococcus aureus, Pseudomonas aeruginosa, Nocardia, or mycobacteria
Fungi: Infection due to Candida, Aspergillus, Zygomycetes, Histoplasma, Coccidioides, or Pneumocystis jirovecii
Parasites: Infection due to Toxoplasma gondii, Strongyloides stercoralis, Cryptosporidium, microsporidia, or Isospora belli
Drugs that can increase heat production
Methylenedioxymethamphetamine (MDMA, or Ecstasy)
Drugs that can trigger fever
Interferons (drugs that are based on substances produced by the immune system and that help block the reproduction of viruses)
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:
In people with an acute fever, certain signs and characteristics are cause for concern. They include
When to see a doctor:
People who have any warning sign 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, exposure to infections, and recent travel. The pattern of the fever rarely helps the doctor make a diagnosis, except a fever that recurs 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 exposure to certain materials or animals is 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).
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:
The physical examination begins with confirmation of fever. Fever is most accurately determined by measuring rectal temperature. Then the doctor does a thorough examination from head to toe to check for a source of infection or evidence of disease.
The need for testing depends on what the doctor finds during the 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 require testing. Exceptions are 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.
People who are at increased risk of infection and people who appear seriously ill often need testing even when findings do not suggest a particular disorder. For such people, doctors often do the following:
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 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.
A fever of unknown origin may be diagnosed when people have a fever of at least 101° F (38.3° C) for several weeks and when extensive investigation does not detect a cause. In such cases, the cause may be an unusual chronic infection or something other than an infection, such as a connective tissue disorder or cancer. 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, 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 and cultured.
Because fever helps the body defend against infection and because fever itself is not dangerous (unless it is higher than about 106° F [41.1° 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.The most effective and widely used antipyretics are acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen, and naproxen. Typically, people may take 650 to 1000 milligrams of acetaminophen every 6 hours (not to exceed 4000 milligrams in 1 day). Alternatively, they may take 400 milligrams of ibuprofen every 6 hours. 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 ≥ 106° F. Sponging with alcohol is avoided because alcohol can be absorbed through the skin and may have harmful effects.
Essentials for Older People
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 significantly 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.
Diagnosis 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. Older people with a blood infection or with abnormal vital signs (such as low blood pressure and a rapid pulse and breathing rate) are admitted to the hospital.
Last full review/revision October 2012 by Allan R. Tunkel, MD, PhD