Fever in Adults

Picture of a woman with fever and high temperature

A fever (also termed pyrexia) is a higher-than-normal body temperature. It is a symptom caused by a wide variety of illnesses. Fevers may occur in anyone at any age; however, this article is specifically addressing fever in adults.

Every one of us has experienced the wave of chills and exhaustion that a fever causes. Fever usually occurs in response to an infection as with the flu, viruses that causes a cold, strep throat bacterial infection, or most infectious diseases, or with inflammation that occurs with tissue injury or disease (such as with some cancers). However, many other causes of fever are possible, including drugs, poisons, heat exposure, injuries or abnormalities to the brain, or disease of the endocrine (hormonal or glandular) system.

A fever rarely comes without other symptoms. It is often accompanied by specific complaints, which may help to identify the illness causing the fever. This can help the doctor determine which treatment is necessary.

• Normal body temperature can vary depending on the individual, the time of day, and even the weather. For most people, a temperature of 98.6 F (Fahrenheit) (37 C or Celsius) is baseline.
• Temperature is usually controlled by the part of the brain called the hypothalamus. The hypothalamus is like a thermostat for the body. It maintains normal temperature through heating mechanisms, such as shivering and increased metabolism, and cooling mechanisms, such as sweating and dilating (opening) blood vessels close to the skin.
• Fever occurs when the body's immune response is triggered by pyrogens (fever-producing substances). Pyrogens usually come from a source outside the body and, in turn, stimulate the production of additional pyrogens inside the body. Pyrogens tell the hypothalamus to increase the temperature set point. In response, our body begins to shiver; our blood vessels constrict (close); we get under the covers in an attempt to reach the new temperature that is higher than our baseline. However, other pyrogens can be produced by the body, usually in response to inflammation; these are referred to as cytokines (also termed endogenous pyrogens).
  • Pyrogens (fever-producing substances) that come from outside the body include the following:
    • Viruses
    • Bacteria
    • Fungi
    • Drugs
    • Toxins

How to Take Your Temperature

Body temperature measurements are usually measured by temperature devices inserted on or into the rectum, mouth, axilla (under the armpit), skin, or ear (ear thermometers). Some devices (laryngoscopes, bronchoscopes, rectal probes) may have temperature-sensing probes that can record temperature continually.

The most common way to measure body temperature was (and still is in many countries) with a mercury thermometer; because of glass breakage and the possibility of subsequent mercury contamination, many developed countries use digital thermometers with disposable probe covers to measure temperature from all of the body sites listed above.

• Disposable temperature-sensitive strips that measure skin temperature are also used.
• Oral temperatures are most commonly measured in adults, but rectal temperatures are the most accurate because environmental factors that increase or decrease temperature measurements have the least effect on the rectal area.
• Rectal temperatures, when compared to oral temperatures taken at the same time, are about 1.8 F (0.6 C) higher. Consequently, an accurate measurement of body temperature (best is rectal core temperature) of 100.4 F (38 C) or above is considered to be a "fever" and the person has a febrile illness.
• A newer option includes a temperature-sensitive infrared device that measures the temperature in the skin by simply rubbing the sensor on the body. These devices can be purchased in most pharmacies.

Dangerous temperatures are high-grade fevers that range from over 104 F to 107 F.

Low-grade fevers range from about 100 F-101 F; 102 F is intermediate grade for adults but a temperature at which adults should seek medical care for an infant (0-6 months).

High-grade fevers range from about 103 F-104 F. Dangerous temperatures are high-grade fevers that range from over 104 F-107 F or higher (extremely high fevers are also termed hyperpyrexia). The preceding fever values may vary somewhat according to the condition and age of the patient, but they offer a reader a way to judge the terms "low," "high," and "dangerous" when they are used in reference to fever in the medical literature.

Consequently, regarding the question of "when to worry" or better, "when to act" about a fever, it is usually considered to be in the case of intermediate- and high-grade fevers. Low-grade fevers that last more than about four to seven days may need investigation by a medical caregiver while persistent fevers (low-, intermediate-, or high-grade) always need investigation.

Other terms are used to describe fever or fever types:

• Prolonged or persistent fever is fever lasting longer than about 10-14 days; these are usually low-grade fevers.
• Acute fever is a sudden onset of an illness that produces the symptom of fever, an increase in the body's temperature set point.
• Constant fever is also termed continuous fever; it is usually low-grade fever and does not change by much (by about 1 degree F over 24 hours).
• Chronic: fever lasts longer than three to four days; some physicians consider intermittent fevers that recur over months to years as "chronic" fevers.
• Intermittent: temperature either varies from normal to fever levels during a single day or fever may occur one day and recur in about one to three days
• Remittent: fevers come and go at regular intervals.
• Hyperpyrexia: fever that is equal to or above 106.7 F; this temperature is too high -- it constitutes a medical emergency for the patient.

In addition, there are well over 40 diseases that have "fever" as part of the disease name (for example, rheumatic fever, scarlet fever, cat scratch fever, Lassa fever, and many more). Each disease has fever as one of its symptoms; countless other conditions may have fever as a symptom.

Cytokines or endogenous (body-generated) pyrogens can cause many of the same features mentioned above. Cytokine release is triggered by inflammation and many immune-mediated diseases. People may have both infectious (also termed exogenous) pyrogens and cytokines generating fevers at the same time, depending on their disease processes. The major cytokines involved in fever generation are interleukins 1 and 6 along with tumor necrosis factor (TNF)-alpha.

Common symptoms can include a runny nose, sore throat, cough, hoarseness, and muscle aches.

Viral Fever

Illnesses caused by viruses are among the most frequent causes of fever in adults. Common symptoms can include a runny nose, sore throat, cough, hoarseness, and muscle aches. Viruses also may cause diarrhea, vomiting, or an upset stomach.

For the most part, these viral illnesses will improve simply with time. Antibiotics will not treat a viral infection. Symptoms can be treated using decongestants and anti-fever medications bought over the counter. If diarrhea or vomiting occurs, then the person needs to be encouraged to drink fluids. Gatorade or sports drinks will help replace lost electrolytes. If fluids are not staying down, then medical care should be sought. Viral illnesses can last as long as one to two weeks.

The influenza virus is a major cause of death and serious illness in the elderly. Symptoms include headaches and muscle and joint aches, as well as the other common viral symptoms, including fever. Vaccines against seasonal influenza as well as H1N1 influenza are available. Also, antiviral medications can be administered to fight the influenza virus immediately after the symptoms start. This illness usually occurs during the winter.

Bacterial Fever

Bacterial illnesses causing fever can affect almost any organ system in the body. They can be treated with antibiotics.

• Central nervous system (brain and spinal cord) infections can cause fever, headache, neck stiffness, or confusion. A person may feel lethargic and irritable, and light may irritate the eyes. This could represent meningitis or a brain infection, so the person with these symptoms should access medical care immediately.
• Lower respiratory system infections, including pneumonia and bronchitis, can cause fever. Symptoms include coughing, difficulty breathing, thick mucus production, and sometimes chest pain.
• Upper respiratory system infections occur in the throat, ears, nose, and sinuses. A runny nose, headache, cough, or sore throat accompanied by a fever may indicate a bacterial infection, but a viral infection is the most common cause.
• Infection of the genitourinary system may cause a person to have a burning sensation when urinating, blood in the urine, the urge to urinate frequently, and back pain along with a fever. This would indicate an infection in the bladder, kidney, or urinary tract. Antibiotics would treat such an infection.
• If the reproductive system is affected, people often see a discharge from the penis or vagina and have pelvic pain along with the fever. Pelvic pain and fever in women may represent pelvic inflammatory disease (PID), which can cause significant damage to the reproductive organs. In this case, the person and any sexual partners should see a physician.
• Gastrointestinal system (digestive system) infections are indicated by diarrhea, vomiting, stomach upset, and sometimes blood in the stool. Blood in the stool can indicate a bacterial infection or other type of serious illness. Abdominal pain may be caused by an infection of the appendix, gallbladder, or liver, and medical care should be accessed.
• The circulatory system (including the heart and lungs) can be invaded by bacteria. There may not be any specific symptoms with the fever. A person may feel body aches, chills, weakness, or confusion. The condition known as sepsis is present when bacteria enter the bloodstream. An infection of a heart valve with resulting inflammation (endocarditis) can occur in people who had heart surgery in the past and in people who use IV drugs. This condition requires hospitalization and immediate treatment with IV antibiotics.
• Skin, the largest organ in our body, can also be the source of a bacterial infection. Redness, swelling, warmth, pus, or pain occurs at the site of the infection. An infection may result from trauma to the skin or even a clogged pore that becomes an abscess. The infection can spread to the soft tissues beneath the skin (cellulitis). Sometimes the infection needs to be drained. Antibiotics are often needed. In addition, skin can react to some toxins by producing a skin rash; for example, the scarlatina rash that can occur after a strep throat infection causes scarlet fever (skin rash is bright red and diffuse, with some skin that develops scaling and desquamation, or skin peeling off).

Fungal Fever

Fungal infections can affect any organ system. Often a physician can identify these infections through a physical examination. Sometimes further testing is required and in rare instances, fungal fevers may require a biopsy to diagnose the infection. An antifungal medication will usually treat the infection.

Animal Exposure Fever

Certain people who work with animals can be exposed to rare bacteria that can cause fevers. In addition to the fever, the person may have chills, headache, and muscle and joint aches. These bacteria can exist in livestock, in unpasteurized dairy products, and in the urine of infected animals.

Travelers' Fever

Anyone who travels, especially outside the United States, may develop fever after exposure to various new foods, toxins, insects, or vaccine-preventable diseases.

The only vaccines required by the U.S. and other countries for travelers at this time are for yellow fever and meningitis; these requirements depend on when and where people travel. Childhood vaccines such as those against measles, mumps, rubella, diphtheria, tetanus, and polio should be current prior to travel. Vaccines against hepatitis A, meningitis, and typhoid can be obtained before people travel to an area where exposure to those diseases is likely. The U.S. Centers for Disease Control and Prevention (CDC) can advise people on the current vaccines that are recommended or required for travel to various countries.

When traveling, consumption of contaminated water, uncooked vegetables, or unpasteurized dairy products can cause a low-grade fever and traveler's diarrhea. Bismuth subsalicylate (Pepto-Bismol), loperamide (Imodium), and certain antibiotics can help reduce symptoms but in some people may prolong the disease. The symptoms and signs of abdominal cramping, nausea, vomiting, headache, and bloating should go away in three to six days. A fever higher than 101 F (38.3 C) or the presence of blood in the stool is an indication to go to a doctor immediately.

Insect bites are a common way that infections are spread in some countries. Malaria is a serious infection that can occur after a mosquito bite. The bitten person may have fevers that come and go every few days. A blood test must be done to make the diagnosis. In certain infected areas, a traveler can take medication to prevent malaria. Lyme disease is spread by the bite of a tick. This is common in areas of the U.S. where the deer tick is found. Any infection caused by an insect bite should be evaluated by a doctor.

Drug Fever

A fever that occurs after starting a new medication, without another source, may be a drug fever. The fever can occur at any time after starting the drug and should go away after the drug is stopped. Some drugs that have been associated with fever include beta-lactam antibiotics, procainamide (Procanbid), isoniazid, alpha-methyldopa, quinidine (Quinaglute Dura-Tabs), and diphenylhydantoin.

• An immediate fever may be caused by an allergic response to the medication or a preservative in the medication.

Blood Clot Fever

Occasionally a blood clot can develop in a person's leg and cause swelling and pain in the calf. Part of this clot may break off and travel to the lungs (pulmonary embolus). This may cause chest pain and trouble breathing. In either case, a person may develop a fever because of inflammation in the blood vessels. A person with any of these symptoms should go to the hospital.

Tumor Fever

Cancer can cause fever in a variety of ways. Sometimes the tumor makes pyrogens, chemicals which cause a fever on their own. Some tumors may become infected. Tumors in the brain may prevent the hypothalamus (the body's thermostat) from properly regulating the body temperature. Many of the medications that a cancer patient takes can cause a fever. Finally, immune systems in cancer patients may be weakened, which makes them prone to various infections.

Environmental Fever

Occasionally, a very high body temperature can be reached when the body becomes overheated. This condition is called hyperthermia. This often occurs with strenuous exercise or when the body is exposed to hot or humid weather. Certain drugs that alter a person's behavior may prevent that person from taking shelter from the heat. People with hyperthermia may be confused, lethargic, or even comatose. They may have an extremely high temperature and may not be able to sweat. Hyperthermia is treated differently than other causes of fever; it is a medical emergency. The affected person must be cooled immediately.

Special Medical Conditions

Many people have medical illnesses that prevent their immune system (defense system) from working normally. This may make it easier for a fever-causing infection to invade their body. Depending on the illness, it may be difficult to find the source of the fever. A fever in a person with limited ability to fight off infection can be very dangerous. The collagen vascular diseases and autoimmune diseases (for example, systemic lupus erythematosus, rheumatoid arthritis, polyarteritis nodosa) may be associated with fever. Many diseases of the immune system produce fever, because of inflammation.

The following are causes of a weakened immune system:

• Cancer
• Cancer treatments
• Immunosuppressive medication, such as for organ transplants
• Steroid therapy for a long time
• HIV
• Age older than 65
• Absence of the spleen (after surgical removal of the spleen)
• Sarcoidosis (a condition characterized by an unusual form of inflammation, leading to the formation of so-called granulomas, that may occur anywhere in the body)
• Lupus
• Malnutrition
• Diabetes
• Heavy alcohol or drug use

Any person with one of these illnesses or conditions and a fever should see a doctor or go to a hospital's emergency department quickly. It is important for the proper treatment to be started right away. Quick action may save the person's life.

Another medical condition that involves fever is unusual as the cause is unknown or unexplained (although the cause may be discovered at a later date). It is termed FUO (fever of unknown origin). FUOs are defined as a temperature greater than 101 F (38.3 C) on several occasions, with more than three weeks' duration of such febrile illness, and failure to reach a diagnosis despite intensive investigation, which some investigators consider to be one week of inpatient investigation. Eventually, FUOs are found to be caused by infections, cancers, collagen vascular diseases, and numerous miscellaneous diseases such as abscess in organs, obscure parasitic infections, and occult cancers. Unfortunately, some FUO cases defy diagnosis, despite expert evaluations and many tests.

Another special medical condition involves hypothalamus regulation. Neurotransmitters and hormones (for example, thyroid hormones) work through feedback mechanisms to help the hypothalamus function. If this delicate feedback balance is interrupted, the hypothalamus may malfunction in many ways, one of which is to raise core body temperature to fever levels. Thyroid storm (also termed thyrotoxicosis) is a medical emergency in which fevers reach about 105.8 F (41 C).

When to Call or See a Doctor (or When to Worry About a Fever)

A fever has many possible causes. Most commonly, a fever is part of a viral infection that will go away on its own. However, there are some reasons to be concerned or worried about a fever; do not hesitate to call or see a doctor for any high fevers; the following is a "when to be worried" list that lists some symptoms and signs that indicate that people should seek medical care.

• Call the doctor if any of these conditions exists:
  • If the temperature is 103 F (39.4 C) or greater (fever is too high)
  • If the fever lasts more than seven days
  • If the fever symptoms get worse (if fever is increasing toward 103 F or 39.4 C)
• Call the doctor or consider going to an emergency center immediately if any of the following symptoms occur with any fever.
  • Confusion or excessive sleepiness
  • Stiff neck
  • Severe headache
  • Sore throat, especially with difficulty swallowing or if the person is drooling
  • Rash
  • Chest pain
  • Trouble breathing
  • Repeated vomiting
  • Abdominal pain
  • Blood in stool
  • Pain with urination
  • Leg swelling
  • Red, hot, or swollen area of skin
• People with serious medical illnesses, such as cancer or HIV, may not show some or any of these warning signs. Mild symptoms with fever in this patient population should be discussed with the doctor to prevent them from progressing into more serious infections or other conditions.

When to Go to the Hospital

Certain illnesses that occur with a fever can be life threatening. Under these conditions, the person should go immediately to a hospital's emergency department:

• Meningitis is life-threatening and highly contagious if caused by certain bacteria. If a person has the combination of a fever, severe headache, and stiff neck, he or she should be taken to the emergency department immediately.
• A person with difficulty breathing or chest pain and a fever should go immediately to the emergency department or call for emergency medical transport.
• If a person has a fever and blood in the stool, urine, or mucus, he or she should seek emergency medical help.
• A person who has a fever and is very agitated or confused with no obvious reason should be transported to the emergency department.
• Any person whose immune system is weakened (for example, people with cancer or AIDS) should call their doctor or go to the emergency department immediately if a fever develops. (See special medical conditions.)
• Hyperthermia is an emergency. Call for emergency medical transport if a person has a temperature equal to or greater than 104 F (40 C), is confused, or is not responding to verbal stimuli or commands.

A health care professional will ask many questions in an effort to find the source of the fever:

• When the fever started
• What other symptoms occurred
• The person's immunization status
• Any recent travel
• Any exposures to sick people at work or at home
• Any medications taken or illicit drug use
• Exposure to animals
• Sexual history
• Recent surgeries
• Any underlying medical illnesses
• Allergies

A very thorough physical examination will be done in an effort to find the source of the fever. After the history is taken and physical examination is performed, the physician may know the cause of the fever. If the physician is not sure at this point, he or she may order certain tests to help make the diagnosis. Examples of diagnostic tests that may be ordered are as follows:

• a blood test to measure the white blood cell count,
• strep throat culture,
• sputum sample,
• blood culture,
• urine analysis,
• urine culture,
• stool sample,
• spinal tap (lumbar puncture),
• X-ray films or CT scan,
• liver function tests,
• thyroid function tests.

Based on the results of these tests, the physician usually will be able to find the cause of the fever. More specific tests, including imaging tests, may be done if needed if the initial tests do not suggest a cause for the fevers.

FUOs (fevers of unknown origin) are challenging, and often specialists need to be involved to help determine what further diagnostic testing may be needed (for example, endoscopy, PET scanning, echocardiography, or radionucleotide studies).

People can make the diagnosis of fever at home by taking a person's temperature with a thermometer, and there multiple ways to bring a fever down.

There are several ways to bring down (reduce) a fever. In general, a fever can be reduced with ibuprofen (Advil, Motrin, Nuprin) or acetaminophen (Tylenol and others). Both medications help control pain and reduce fever. Alternating doses of each will also work and prevent accidental overdose of one drug. At times, a combination of both acetaminophen and ibuprofen will be needed to stop the fever. Cool bath water or cool towels applied to a person's skin may also help reduce fevers; cool fluids taken orally will also rehydrate and cool a person.

Aspirin is not the first choice drug for fever reduction; it should not be used in children. Aspirin may be toxic in large doses in adults or cause Reye's syndrome in children. Do not give aspirin to individuals 18 years or younger unless directed by a physician to give a specific dose.

• Ibuprofen stops the hypothalamus from raising the body temperature. It comes in 200 mg tablets purchased over the counter at a drugstore. It's OK to take one to two tablets every four hours to decrease one's temperature. Use the lowest possible effective dose. Children's doses are based on the child's weight.
  • Side effects of ibuprofen include nausea and vomiting, which may be prevented if the medication is taken with food. Rare side effects include diarrhea, constipation, heartburn, and stomach pain. People with stomach ulcers or kidney disease, pregnant women, and those with an aspirin allergy should avoid ibuprofen.
• Acetaminophen is also effective at reducing a fever. It comes in 325 mg tablets or 500 mg tablets over the counter. It may also be available in liquid formulations. Again, one to two tablets every four hours should be used to eliminate a fever. Like many other medications, children's doses are based on the child's weight. The total dose should not be more than 3 grams (equivalent to six of the 500 mg tablets) per 24 hours in adults.
  • Side effects are rare, but some people are allergic to the medication. Extremely large doses (overdose) may cause liver failure. Therefore, people with liver disease and chronic alcohol users should avoid this medication.
  • Common brand names of acetaminophen are Aspirin Free Anacin, Feverall, Genapap, Panadol, Tempra, and Tylenol. Read the product label for specific ingredients described as acetaminophen. Many other drugs contain acetaminophen in combination with other drugs so medicines should be checked to ensure that the total dose, even with combination medicines, should not exceed 3 grams in 24 hours.
• A fever can cause anyone to become very dehydrated. Drink plenty of fluids. Attempts to cool the skin may only make a person more uncomfortable. This may also cause shivering, which will actually increase the body temperature if the fever is being caused by an infection. Further therapy depends on the cause of the fever and the accompanying symptoms. Basic cold symptoms can be treated with over-the-counter medications.
• If the fever is caused by exposure to hot weather or overexertion (for example, heat stroke, hyperthermia, and heat exhaustion), the technique is different from treating any other fever. Neither acetaminophen nor ibuprofen will be effective. The person needs to be cooled immediately. If the person is confused or unconscious, seek emergency medical help immediately. While waiting for help, remove the person from the hot environment and remove his or her clothes. The body should be cooled with a wet sponge, and a fan should be directed over the person.

The treatment of a fever (or how to break the fever) depends on its cause. In most cases, except in hyperthermia, acetaminophen or ibuprofen can be given to lower the temperature (see home remedies above). Fluids may be given by mouth or IV to prevent dehydration, if necessary.

• Viral illnesses usually resolve without medical treatment. However, medications to help with specific symptoms can be given. These may include medications to lower fever, help with congestion, soothe a sore throat, or control a runny nose. Viruses that cause vomiting and diarrhea may require IV fluids and medications to slow down the diarrhea and stop nausea. A few viral illnesses can be treated with antiviral medications. Herpes and the influenza virus are examples.
• Bacterial illnesses require a specific antibiotic that depends on the type of bacteria found or where it is located in the body. The physician will determine whether the person is admitted to the hospital or sent home. This decision is based on the illness and the person's overall health status.
• Most fungal infections can be treated with an antifungal medication.
• Drug-induced fever is eliminated when the medication is stopped.
• A blood clot requires admission to the hospital and blood-thinner medications.
• Any person with an illness that inhibits the immune system will be evaluated closely and usually admitted to the hospital.
• Environmental heat exposure requires aggressive cooling in the emergency department. The person's clothes will be removed, a cooling fan and cool mist will be used, and his or her vital signs will be monitored closely. Hyperthermic people will be admitted to the hospital.

Thyroid storm is treated by blocking hormone production with drugs like methimazole (Northyx, Tapazole) and iodine to block hormone release plus propranolol (Inderal) to further block the effects of thyroid hormones.

Most fevers will go away in a few days with the appropriate treatment. It is important to follow up with a health care professional to be sure the cause of the fever is treated correctly. This may be done in a few days to weeks after the initial visit, depending on the cause.

If symptoms worsen, if the fever continues for more than three days despite treatment, or if the fever lasts longer than a week without treatment, see a doctor immediately.

Follow-up is very important especially for people with fevers due to cancer, drug-induced fevers, infectious causes like tuberculosis, FUOs, or hormone problems, since these people may experience relapses and repeated treatments. In some cases, hospitalization may be required.

Most fevers come from an infection. Individuals can help prevent the spread of infection and thus prevent fever.

• The best way to prevent the spread of infection is to wash the hands frequently and avoid touching the face or mouth as much as possible.
• Keep the home and work environment clean.
• Avoid direct contact with sick people.
• Do not share cups or utensils, towels or clothing, especially if they are not clean.
• Wear appropriate protective clothing and equipment when working with animals.
• Make sure immunizations are current and get the appropriate preventive medication and immunizations when necessary if traveling to another country.
• Do not use illegal drugs.
• During strenuous exercise, stay well hydrated, wear cool clothing, take frequent breaks, and cool down after the workout. Avoid use of alcohol and drugs that can alter behavior and judgment, and don't prevent a person from seeking shelter from the heat.

• In most cases, a fever will come and go without much intervention from a doctor. If a specific cause for fever is found, then the doctor can prescribe the appropriate medication and treat the illness. Occasionally, a second antibiotic, an antifungal medication, or other drug will be needed. Usually, with the appropriate therapy an infection will resolve and the person will return to a normal temperature.
• In some cases, a fever can be life-threatening. This is often seen in people with poor immune systems, certain types of meningitis, and severe abdominal pain. Pneumonia with fever can be life threatening in an older person. Any infection in which the source is not found can continue to get worse and become very dangerous.
• Severe hyperthermia can cause a coma, brain damage, or even death. Usually, if the cause of the fever is diagnosed quickly and treated appropriately, the prognosis is good, but the prognosis is poorer if there are diagnostic and treatment delays and organs become progressively damaged.