Fever In Adults

  Where Does Fever Come From?

     How Do I Measure Temperature?
     What Other Symptoms Typically Accompany Fever?
     Types of Illnesses Cause Fever, and What Are Their Symptoms?
     How Do I Treat Fever?
     Is There a Downside to Treating a Fever?
     What Should I Worry About? When Should I Call My Doctor?
     What Might the Doctor Do?
     How Do I Prevent Fever?

          Normal body temperature is usually cited as 98.6 degrees Fahrenheit, as measured by a thermometer placed under the tongue (“oral temperature”).  “Normal” temperature, however, varies throughout the day and according to your level of activity; it is usually lowest early in the morning and highest in late afternoon.  Feveris an elevation of body temperature above that normal range and is the human body’s normal and healthy reaction to infection. It is caused by a complex chain of biochemical and cellular reactions that are launched to fight infection.  Elevation of body temperature associated with strenuous exercise or with exposure to hot weather is not considered a fever.

          Fever is a symptom, not a disease.  The magnitude of fever does not necessarily correlate to the severity of infection, as fever may be very high with a brief viral illness and lower (or sometimes the body temperature is actually abnormally low) in the face of a life-threatening infection.  It is generally felt that a fever up to around 102 degrees Fahrenheit (oral) can help the body fight infection (1).  Under most circumstances (exceptions include patients who have an immune deficiency, such as patients with HIV/AIDS or those taking chronic steroid therapy or cancer chemotherapy), a fever of 103-104 degrees Fahrenheit can be tolerated for a day or two, although such a high fever is associated with malaise and fatigue.  Children tend to have higher fevers than adults.  This Knol discusses fever in adults; fever in children is discussed separately.

Fever In Adults

Where Does Fever Come From?


          Body temperature is controlled by an almond-sized organ deep in the brain called the hypothalamus (FIGURE 1).  The hypothalamus controls metabolic processes in the body (such as body temperature, hunger, thirst, fatigue, and stress reactions) by releasing tiny amounts of chemicals that act on other body organs either directly or indirectly.  With respect to body temperature, the hypothalamus acts as a thermostat; it helps maintain body warmth when someone is exposed to cold temperatures by causing shivering and an increased metabolic rate.  When someone is hot (from a fever, from exercise, or from hot weather), it causes sweating and dilation (opening up) of blood vessels in the skin, allowing dissipation of heat. 

          Fever results when the hypothalamus detects the presence of pyrogens, or fever-inducing substances. Pyrogens are usually foreign substances to the body, such as bacteria, viruses, parasites, drugs, or toxins. Other abnormal cells, such as some tumor cells and inflammatory cells in people with diseases such as rheumatoid arthritis, Crohn’s disease, or temporal arteritis, are sometimes viewed by the body as “foreign” and pyrogenic.  The presence of these pyrogens activates other “native” (not foreign) infection-fighting pyrogens such as white blood cells inside the body, and the accumulation of these substances signals the hypothalamus to raise the temperature on the body’s thermostat.  In response, the patient feels cold and his body shivers, blood vessels in the skin constrict (tighten down) to retain heat, and he seeks warm clothing or bed covers, all in an attempt to reach the new target temperature on the thermostat.  That new, higher temperature represents a higher metabolic level for the body, assisting the body’s fight against the infection.

          Once the higher temperature is reached, the patient begins to feel hot.  Clothing and bed covers are shed, and the temperature begins to drop back towards normal.  The patient may begin to sweat and the blood vessels in the skin dilate, leading to rosy cheeks and release of heat.  Fever medications, also called “anti-pyretics” such as acetaminophen and aspirin, work by interfering with the function of the hypothalamus.  These drugs therefore inhibit the body’s natural response to infection (see “Is There a Downside to Treating a Fever?,” below).


How Do I Measure Temperature?

          Body temperature can be measured by glass thermometers or plastic analog or digital thermometers placed under the tongue (oral), into the rectum (rectal), or into the armpit (axillary).  There are also digital thermometers that measure temperature inside the ear (tympanic) or on the forehead.  Axillary temperatures are not considered reliable by physicians.  Rectal or tympanic temperatures are closer to the body’s true “core” temperature, but temperature in adults is most commonly and conveniently measured orally.  Oral temperature is measured by placing the thermometer under the tongue, and holding it there for three to five minutes (or until a digital thermometer signals that it is ready) without talking and preferably while breathing through the nose. Avoid measuring oral temperature within 15 minutes after drinking a hot or cold beverage.  Rectal temperatures are measured by placing the probe one-half to one inch inside the rectum.  Rectal temperatures are always about one degree higher than oral temps, and a fever is defined as a rectal temperature greater than 100.4 degrees Fahrenheit.

          If no thermometer is available, feeling the skin temperature on the forehead or the side of the neck may give a relative sense of whether or not fever is present.  Flushing of the skin, shaking chills, fatigue, and body aches often accompany fever and may be suggestive in the absence of actual temperature measurement.

What Other Symptoms Typically Accompany Fever?

          Common general symptoms accompanying a fever in adults include general malaise and weakness, sweating, shivering, muscle and joint aches, loss of appetite, headache, and nausea or an “upset stomach.” With very high fevers (103 to 106 degrees Fahrenheit), more serious symptoms such as confusion, hallucinations, and convulsions (“febrile seizures,” which are much more common in children) may occur. Other, more specific symptoms are determined by the nature (for example, bacteria vs virus) and location (for example, nose and throat vs gastrointestinal tract) of the pyrogen.  The symptoms accompanying fever give physicians clues to the source of the illness and the need and urgency for specific treatment.


What Types of Illnesses Cause Fever, and What Are Their Specific Symptoms?

          The most common source of fever in otherwise healthy adults is a viral infection (1).  Viruses can cause “URIs,” or upper respiratory infections, associated with fever, sore throat, nasal congestion or runny nose, cough, and muscle aches.  Other viruses cause gastrointestinal illnesses, marked by fever, nausea with or without vomiting, diarrhea, abdominal pain, and malaise.  Chicken pox is caused by the varicella virus, and in addition to typical skin lesions, may cause pneumonia (lung disease) and generally more severe illness in adults than in children.  Normally, such viral illnesses resolve on their own, over time (they may last as long as a week or two), with what physicians call “supportive care.”  Supportive care includes rest, lots of noncaffeinated fluids to drink, and over-the-counter medications such as cough syrup, decongestants, or bismuth subsalicylate (drugs used to treat upsets of the stomach and gastrointestinal tract).  Viral infections, by definition, do not improve with antibiotics and, in fact. patients with viral illness who are treated with antibiotics may end up feeling worse than they did before therapy.

          There are of course more serious viral infections.  Influenza (flu) is caused by a virus and can be a significant source of illness and even death, especially in the elderly.  Symptoms of influenza include fever, headaches, muscle aches, joint aches, and severe malaise.  An influenza vaccine is offered each year to people over 50 years of age and others at high risk of exposure to the flu; because the incidence of flu typically peaks in the winter, vaccination is usually given in the autumn.  Other important viruses that can cause fever include mononucleosis (which infects the lymph glands and spleen), hepatitis (liver), viral meningitis (fluid around the brain and spinal cord), and viral encephalitis (brain).  Although fever is nearly always present with one of these serious illnesses, other symptoms typically cause more concern and prompt consultation with a physician.

          Bacterial infections can occur in most organs and tissues in the body and are usually more serious than viral illnesses, even though most are easily treatable with antibiotics.  Fever is a typical feature of bacterial infections.  Fever associated with a bacterial infection of the central nervous system (brain and spinal cord) typically is accompanied by headache, stiff neck, and changes in behavior such as lethargy or confusion.  These symptoms should prompt urgent consultation with a physician. The skin, which is the largest organ of the body, can be infected by bacteria.  Usually there is a clear point of bacterial entry, such as a cut or scrape.  Skin infections are characterized by fever accompanied by pain, redness, swelling, and warmth (FIGURE 2, which is reprinted by permission from UCSD "Catalog of Clinical Images" at http://meded.ucsd.edu/clinicalimg/index.html with permission of Charlie Goldberg, MD).

Sometimes pus drains from the area.  Antibiotics are used to treat such illnesses, which are called “cellulitis”(2).  An incision is sometimes required to drain the infection from the skin and underlying tissues.

          The respiratory and gastrointestinal tracts are common locations for bacterial infections that cause fever.  Bronchitis and pneumonia cause cough and difficulty breathing in addition to fever, while gastrointestinal infections are associated with fever, abdominal pain, loss of appetite, vomiting, and/or diarrhea.  Urinary tract infections may cause fever, flank pain, nausea and vomiting, and burning and frequent urination.  Bacterial infections of the genital tract can cause, in addition to fever, a discharge from the penis or vagina.  Treatment should be sought both to cure the infection and also to prevent its spread to a partner.  The bottom line about bacterial infections is that, although fever is often part of the general illness, there are specific symptoms that can direct a caregiver to a specific cause and thence to specific antibacterial therapy.

          Fever can also result from other, less common causes.  Some fungal infections are associated with fever; while often difficult to diagnose, effective antifungal therapy is usually curative.  High fever also is a hallmark of some serious parasitic infections such as malaria.  Fever also can result from some vaccinations (such as hepatitis B and influenza), from some medications (“drug fever,” often caused by antibiotics after more than a week of use, or by antiseizure or antidepressant medications), and from cancer (“tumor fever”). Each of these fevers is complicated to evaluate and will require input from a physician.  Physicians define “fever of unknown origin” (FUO) as a persistent fever without a clear cause.  While concerning to both patient and physician, FUO is usually benign and self-limited, and often no cause is ever clearly identified (3).

How Do I Treat Fever?

         Fever can be self-treated with acetaminophen, ibuprofen, or aspirin.  Aspirin is not recommended in children, and is not optimal for use in adults.  Both acetaminophen and ibuprofen can also help reduce the aches and pains that often accompany fever.  The adult dose of acetaminophen for fever is 325-650 milligrams every 4-6 hours.  Acetaminophen (Tylenol®, Panadol®, Feverall®, etc.) should be avoided by patients with liver disease.  Ibuprofen is available over-the-counter in 200 milligram tablets (Advil®, Nuprin®, Motrin®, etc.).  The antipyretic dose of ibuprofen in adults in 200-400 milligrams every 4-6 hours.  Ibuprofen should be avoided by patients who are pregnant, have an aspirin allergy, have stomach ulcers, or have kidney disease.  It may be beneficial to alternate or even combine acetaminophen and ibuprofen. 

          General treatment measures that might reduce an elevated body temperature and make the patient feel better overall include rest, oral hydration (with Gatorade® or other noncaffeinated hydration fluids), and over-the-counter medications for specific fever-related symptoms, such as decongestants, cough remedies, or anti-diarrhea medications.

Is There a Downside to Treating a Fever?

          This is an intriguing question for which the answer is not entirely clear.  What is known is that the answer varies by type of illness.  Acetaminophen and aspirin do not prolong the duration of an upper respiratory infection, although they do appear to prolong the duration of the contagious interval and may worsen nasal congestion.  In a small study, acetaminophen did not affect the overall condition of children with chicken pox, but it did prolong the time until scabbing over of lesions (chicken pox is considered contagious until all lesions are scabbed over).  Another study seemed to indicate that patients with influenza A treated with antipyretics had longer courses of illness than those who were not treated, although it also appeared that it was sicker patients – with higher fevers – who took the acetaminophen, so it may be that those patients were destined to remain ill longer, anyway.  What has not been studied well in regards to this question is the balance between symptomatic relief – reduction of the fever itself – and duration of illness; that is, even if patients are ill a day or two longer, do they feel better sooner with the fever treatment?  The thinking behind this question is that, if fever is a natural response to infection, and if the higher metabolic rate associated with fever helps the body fight infection, should we try to suppress fever and interfere with that immune response?  The viruses that cause upper respiratory infections, for example, seem to thrive at cool temperatures.  In summary, however, available data at the present time suggest that there is little if any significant downside to the use of antipyretic medications in patients with fever associated with acute illness (4).

What Should I Worry About? When Should I Call My Doctor?

          If your fever is more than 103 degrees Fahrenheit or has persisted for more than three days, you should call your doctor.  Your doctor should be notified more urgently (or you should go to the nearest hospital emergency department) if you have fever and any of these symptoms:

·         mental confusion or changes in behavior

·         skin rash that is spreading (FIGURE 3 below, reprinted with permission from Annals of Emergency Medicine), or a localized area of the skin that is red, hot, and swollen

·         persistent shortness of breath

·         extreme listlessness or irritability

·         stiff neck muscles and pain upon bending the neck forward

·         severe abdominal pain, persistent vomiting, or diarrhea, with or without blood in stool or vomitus

·         pain when urinating

·         severe swelling in the throat with difficulty swallowing or breathing

·         leg swelling

·         immune compromise, such as HIV/AIDS, chronic corticosteroid therapy, chronic immune suppression therapy (such as organ transplant patients), chemotherapy for cancer, malnutrition, chronic drug or alcohol abuse, or diabetes . . . regardless of other symptoms (5)

What Might the Doctor Do?

          The physician’s first and primary goal in assessing and managing fever is to make certain that the patient does not have a life-threatening illness.  The types of very serious infections that are typically marked by very high fever include meningitis, encephalitis, pneumonia, appendicitis or other intra-abdominal infection, heart valve infection, blood clots in the lungs, and malaria.  The physician will first take a history of the patient’s possible exposure to pyrogens.  Has the patient been in close proximity to another ill patient with a fever?  Does the patient have any immune compromise?  Is he or she taking any new medications or recently received a vaccination?  Has the patient traveled to an area where high-fever diseases such as malaria are more common, or been outdoors in an area where mosquito or tick bites are common?  Has the patient had unexplained fevers in the past?  Has he or she had a surgical procedure recently?  Has there been any unusual exposure to domesticated or wild animals?

          Next, a thorough history of the fever itself will be sought.  How high has the temperature been?  Does it stay elevated despite the use of acetaminophen or ibuprofen?  How many days has the fever been present?  Is there any pattern to the occurrence of fever? 

The physician’s next focus is on the patient’s other, non-fever, symptoms.  Symptoms that prompt immediate concern and thorough evaluation include headache, stiff neck, new rashes, sensitivity of the eyes to light, persistent vomiting and/or diarrhea, and coughing up blood.  What was the time course of these symptoms?  Did their appearance precede or follow the development of fever?  Are these symptoms getting worse?  Are there sweats associated with the fever, and do these occur throughout the day or only at night? The answers to these questions help guide the physician’s thinking; if the fever is high, generalized symptoms (aches, pains, malaise, fatigue) are not prominent, and the specific complaints are limited to the throat, chest, or abdomen, then bacterial infection is relatively more likely and antibiotics may be needed.  If the fever is lower (100.4-101.5 degrees Fahrenheit), systemic complaints are more prominent, and specific complaints are less apparent, then a viral illness is relatively more likely and antibiotics are not expected to be helpful. 

          Next, the physician will inquire about the patient’s overall health and previous illnesses.  The primary focus will be on the patient’s immune status.  Is the patient older than 65 years, even if relatively healthy (6)? Does the patient have a history of an immune-compromising illness, such as HIV/AIDS, diabetes, sarcoidosis, or lupus?  Does the patient have cancer, or has his spleen been removed?  Does the patient take medications that compromise the immune system, such as corticosteroids for emphysema or asthma, or biologic treatments such as Remicade® or Amevive®?  If the answer to any of these questions is yes, then antibiotic therapy is more likely to be prescribed.

          A thorough physical examination will be performed to try to find the source of the fever.  If after these questions and examination the cause of the fever has not been definitely identified, then any of a wide variety of tests may be conducted.  These may include blood tests to determine the intensity of the body’s response to infection; culture samples of the throat, sputum, or mucous and fluids from other sources; x-rays, including such tests as computed tomography (CT) scans; urine tests; and lumbar puncture (“spinal tap”) (FIGURE 4).  These tests not only confirm the presence of absence of infection, but may also assist the physician in choosing a specific therapy beyond treatment of fever.  Such therapy may include antibiotics; antiviral, antifungal, or antiparasitic therapy; surgery; or supportive care only.

How Do I Prevent Fever?

          Because most fevers result from infection, general measures that reduce the spread of infection may also reduce the incidence of fever.  These measures include good hand washing practices, avoiding contact as much as possible with ill people, keeping the home and work environment clean, and maintaining good health practices overall such as keeping immunizations up to date and taking appropriate preventive medications and measures when traveling in less developed areas of the world.



1.  Legget J: Approach to fever or suspected infection in the normal host.  In Goldman L, Ausiello DA, eds: Cecil Medicine, 23rd ed, 2007.

2.  Lopez FA: Skin and soft tissue infections.  Infect Dis Clin N Am  2006;20(4):759-72.

3.  Cunha BA: Fever of Unknown Origin: Clinical Overview of Classic and Current Concepts.  Infect Dis Clin N Am 2007;21(4):867-915.

4.  Hudgings L, Safranek S: Do antipyretics prolong febrile illness?  J Fam Pract 2005;53:57-61.

5.  Pizzo PA: Fever in immunocompromised patients.  N Engl J Med  1999;341:893-900.

6.  Htwe TH: Infection in the elderly.  Infect Dis Clin N Am 2007;21(3):711-43.