When health care providers cannot diagnose the cause of a patient's temperature that reaches 101 degrees Fahrenheit on and off for at least three weeks, they call it a fever of unknown origin (FUO). If the fever persists, your health care provider will continue to carry out tests to narrow down the causes. But in 5 to 15 percent of cases, they fail to find the reason for the fever.

Your health care provider may prefer not to give you medication for your fever while it remains undiagnosed. Research suggests that fever helps fight off infections, so treating the fever without knowing the cause might reduce the body's ability to deal with the possible infection. However, providers will prescribe drugs to reduce fever in children who suffer seizures induced by fever. Because a higher temperature increases a person's need for oxygen, your provider may prescribe fever-reducing drugs if you have heart or lung problems.

• Fever of more than 101°F (38.3°C), either continuous or intermittent, for at least two weeks
• Fever above 101°F whose cause remains unknown even after extensive diagnostic testing

By carrying out a series of tests, health care providers try to narrow down the list of possible reasons for a high temperature.

A provider trying to diagnose the cause of a fever of unknown origin must seek out every possible clue. He or she may ask you questions about:

• Your work, because some workplaces contain organisms that can cause fever
• Places you have visited recently. Locations overseas, and even areas in the United States, can harbor diseases that can cause fever.

Your health care provider will also examine you closely, paying particular attention to your skin, eyes, nails, lymph nodes, heart, and abdomen. He or she will also take blood and urine samples. You may have an ultrasound examination, as well as computed tomography (CT scan) and magnetic resonance imaging (MRI). If the cause of the fever still can't be found, your provider may want to inject you with "labeled white blood cells." These are white blood cells that contain a harmless radioactive compound. Once injected, the white blood cells travel to infected parts of your body. The radioactivity allows your provider to see on an X ray just where they have moved and thus locate the infection responsible for your fever. If that fails, your provider may want to perform minor surgery to take biopsy samples of, for example, your liver or bone marrow.

Your health care provider will advise you to rest and drink plenty of fluids, and may even take you off medications for other ailments, because those medications may be causing your fever. If you have a heart or lung condition, or if your child has seizures as a result of the fever, your provider will probably prescribe over-the-counter remedies to bring down the temperature. The most popular are acetaminophen and aspirin.

• Acetaminophen
• Aspirin and other nonsteroidal anti-inflammatory drugs. Avoid aspirin for children and teenagers, as it increases the risk of Reye's syndrome.

In cases of infection, your doctor may also prescribe an antibiotic, antifungal, or antiviral drug, depending on the cause of the infection.

General immune support with nutrition and herbs may alleviate fevers.

• Eliminate alcohol, caffeine, refined foods, and sugar.
• Drink water or electrolyte replacement (sports) drinks.
• Vitamin C (250 to 500 mg two times per day), beta-carotene (15,000 to 50,000 IU per day), and zinc (10 to 30 mg per day) help your immune system work better and reduce inflammation.

Herbs may be used as dried extracts (capsules, powders, teas), glycerites (glycerine extracts), or tinctures (alcohol extracts). Unless otherwise indicated, teas should be made with 1 tsp. herb per cup of hot water. Steep covered 5 to 10 minutes for leaf or flowers, and 10 to 20 minutes for roots. Drink 2 to 4 cups per day.

The following herbs may be helpful in reducing fever and improving immune response: coneflower (Echinacea purpurea), yarrow (Achillea millefolium), white willow bark (Salix alba), lemon balm (Melissa officinalis), spearmint (Mentha spicata), catnip (Nepeta cateria), and elder (Sambucus nigra). Combine 1 part coneflower and 1 part white willow bark with equal parts of two or more herbs. Drink 3 to 4 cups per day, 2 to 4 oz. three to four times per day for children.

Although very few studies have examined the effectiveness of specific homeopathic therapies, professional homeopaths may consider the following remedies for the treatment of fevers based on their knowledge and experience. Before prescribing a remedy, homeopaths take into account a person's constitutional type. A constitutional type is defined as a person's physical, emotional, and psychological makeup. An experienced homeopath assesses all of these factors when determining the most appropriate treatment for each individual.

• Aconitum -- for fever that comes on suddenly and alternates with chills, heat, and flushing of the face; the individual may be anxious and crave cold drinks
• Apis mellifica -- for fever associated with alternating bouts of wet (sweating) and dry body heat
• Belladonna -- for sudden onset of high fever with hot, red face, glassy eyes, lack of thirst, and hot body with cold hands
• Bryonia -- for fever with symptoms that are aggravated by the slightest movement
• Ferrum phosphoricum -- for the first stages of a fever with a slow onset; this remedy is generally used if Belladonna is ineffective

Acupuncture may be helpful in supporting immune function.

Fever can be dangerous if you are pregnant. Nutritional, herbal, and homeopathic treatments for fevers are generally safe in pregnancy, yet use with caution.

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