Fever of unknown origin, Hyperthermia, Febrile disease

What is fever?

Why do you get fevers when you're feeling sickly? Why do you get chills when you're burning up? The answer has to do with your body's thermostat constantly readjusting itself.

Fever is the general term for any abnormal rise in your temperature above 100°F (or 37.8°C). When your body is fighting an infection or an injury, the built-in thermostat in your brain manufactures white blood cells, your first line of defense. When these cells kick into action, they release a chemical that causes your body temperature to rise, and this causes your fever. You may sweat to release the heat your body has built up as your blood moves throughout your body. Or you may also get the chills as your temperature begins to decline and reach normal levels again.

As common as a fever may be, you should not ignore it. It's almost always the symptom of a variety of underlying conditions, a few of which may threaten your life. Also, your temperature pattern (high or low-grade fever) doesn't necessarily indicate what you may be suffering from. Children may have higher temperatures than adults, while older people may have lower temperatures.

One other thing... remember to supply your body with extra fluids to lower your temperature and help replace fluids you've lost through sweating.

Why does fever occur?

Your brain's thermostat is called the hypothalamus. For most people, it's set at 98.6°F. When an infectious organism - a cold germ, a virus, an allergic reaction, a drug interaction, etc. - invades your body, your white blood cells release a pyrogen, a substance that causes your thermostat to "reset" itself higher.

These pyrogens can be sent to your brain from inside your body or from invading organisms from outside your body (which, in turn, cause your body to release its own pyrogens). Other sources of temperature rise include allergic reactions, cancerous cells, and inflammations.

As your brain redirects blood to areas that need help, your temperature rises to try to reduce the loss of heat. When your white blood cells begin to defeat the invading organisms, you may begin to sweat.

How Common Are Fevers?

Almost everyone has a fever at one time or another. Expectant mothers can get fevers and pass the underlying infection along to the babies they're carrying, which may cause birth defects during the first three months of pregnancy.

What are the possible causes of fever?

There are many possible causes of fever, including:

  • Bacteria
  • Fungus
  • Parasite
  • Virus (pneumonia, flu, mononucleosis, and others)
  • Hormonal change (menopause, hyperthyroidism, pheochromocytoma)
  • Allergic drug reaction
  • Cancer (Hodgkin's disease, leukemia)
  • Chronic infections (tuberculosis)
  • Gastrointestinal disease (inflammatory bowel disease, hepatitis, appendicitis, gall bladder disease)
  • Exotic diseases (malaria)
  • Heart or kidney problems
  • Central nervous system disease (meningitis, encephalitis)
  • Injury
  • Excess sun exposure

Associated symptoms of fever

Your body sends off a number of messages that your fever is the beginning of a more severe problem by responding with the following symptoms: chills, sweating, weight loss, bone pain, rashes; upper torso problems (headache, ear pain, sore throat, stiff neck, chest pain); breathing problems (cough, labored breathing); or abdominal area problems (cramps, urinary pain or frequency, dark urine).

Diagnosing the Underlying Cause

Fever may be a symptom of an underlying condition, disease, or disorder. Some of these disorders have certain characteristic symptoms in addition to exceptionally high fevers.

The following conditions may include fever among their symptoms, but the absence of a high fever should not lead you to conclude that you do not have a more serious disorder. You should not attempt to diagnose yourself with a medical condition, even if your symptoms match those characteristic of a certain disorder. Many disorders have overlapping or look-alike symptoms. If your symptoms concern you, the best thing to do is to seek medical advice. In order to understand your symptoms and reach a diagnosis, your doctor will consider your medical history, what symptoms you have, and the results of a physical examination and laboratory tests.

Fever unaccompanied by other symptoms may be due to:

  • Bacterial infections
  • Malaria or other parasitic infections
  • Wearing warm clothing as your temperature was taken
  • Cancer
  • Mycobacterial and fungal infection

Other symptoms accompanying your fever may help to determine the possible underlying disease. If your fever is accompanied by:

  • Sore throat: You may have an upper respiratory illness, including the common cold.
  • Fatigue: It may be due to mononucleosis, cancer, tuberculosis, or possibly AIDS
  • Cough: You may have pneumonia, tuberculosis, or measles
  • Muscle aches and abdominal pains: This may be due to flu, appendicitis, etc.
  • Headache: This may be due to meningitis (inflammation of the brain and spinal cord membranes) or encephalitis (inflammation of the brain tissue)
  • Bone pain: This may be due to osteomyelitis (bacterial or fungal infection of the bone)
  • Joint pain: This may be due to rheumatic fever and juvenile rheumatoid arthritis
  • Heart murmurs: This may be due to bacterial endocarditis (inflammation of heart tissues)
  • Rash: This could be a result of a drug interaction, Rocky Mountain spotted fever, roseola (a viral infection), or measles

Diagnostic Procedures

If your doctor believes your fever may be related to another condition, he or she will attempt to diagnose the underlying disorder by performing a physical exam or ordering various procedures such as blood tests, cultures, X-rays, or a tissue biopsy.

Fever Treatment & Management


When a child or teen has a fever, please be aware that aspirin may be lethal because it can cause the sometimes-fatal Reye's syndrome. For this age group, acetaminophen (ingredient in Tylenol-like medicines) is more appropriate. Consult a physician immediately.

For most people with fever, the goal of treatment is to lower body temperature. You can take a number of steps on your own to lower your fever, including taking aspirin or acetaminophen (the two most commonly used over-the-counter drugs for lowering temperature). In addition, it is important to determine the cause of the fever so that appropriate treatment can be applied.

Drugs most commonly prescribed (fever reducers)

  • Aspirin (except in children)
  • Acetaminophen (Tylenol)
  • Ibuprofen (Advil)

Dietary modifications

  • Increase fluid and electrolyte intake

Other non-drug treatment

  • Sponge or bathe with lukewarm water (not cold water, which can cause shaking chills)
  • Increase heat loss by removing excess clothing or blanket
  • Keep surroundings cool

Herbal Remedies for Fever

  • White willow bark: This herb is a source of natural aspirin, along with meadowsweet and wintergreen. The herbs contain a compound (salicin) that is a natural precursor of aspirin. A century ago, when Bayer developed aspirin, company scientists began with extracts of these herbs. Like aspirin, teas or tinctures of these herbs help reduce fever.
  • Echinacea: This is an immune-boosting herb. Fever is one of the body's defenses against illness. If you're feverish, your immune system is fighting to make you well. Support your immune system with echinacea. Take a tablespoon of tincture in water or juice three times a day. (Echinacea may cause temporary tingling or numbing of the tongue, which is harmless.)

Self Care & Prevention

No matter what measures you take, it is unlikely that you can prevent fever altogether. Fever is not always a bad thing. If your body is fighting an infection, your temperature may rise. If you've had an injury, or have a heart problem, ear problem, or other condition, you may get a fever as well.

  • Drink lots of nonalcoholic fluids. Fever causes sweating, which is dehydrating. Even minor dehydration can cause some loss of coordination and mental sharpness, which adds to the discomfort of fever. Sip water, tea, and juices throughout the day, and snack on juicy fruits. For meals, try soup.
  • Soak. A tepid bath or sponge bath can help reduce a fever. As the water evaporates from your skin, it has a cooling effect.
  • Rest. The lethargy you feel when you have a fever is your body's way of telling you to take it easy.

Additional Information

Seek Medical Attention

Consult with or see your doctor if you have the following:

  • An unexplained fever lasting several days
  • Fever higher than 103°F, which may cause dehydration, nausea, convulsions, and/or headache
  • Fever with abdominal pain or cramping that lasts more than three days
  • Fever with unexplained weight loss
  • Fever with pain more severe than anything you've experienced in the past
  • Fever with lethargy, vomiting, diarrhea, or other unusual symptoms
  • Fever with rapidly changing symptoms