Fever of Unknown Origin

Fever of "unknown origin" is a fever of at least 101° F (38.3° C) or greater, and that has gone on daily for three weeks. It is a fever which has a hidden cause. Fever is a higher-than-normal body temperature. Normal temperature is usually defined as 98.6° F or 37° C. Fever is a symptom, not a disease. A fever may mean that there is something else going on in the body that is causing it.


Fever can be caused by many conditions, including:

  • Infections.

  • Tissue injuries.

  • Medicines.

  • Different diseases.

  • Being in hot surroundings.

  • Tumors or cancers (this is a rare cause).


The signs and symptoms of a fever depend on the cause. At first, a fever can cause a chill. When the brain raises the body's "thermostat," the body responds by shivering to raise the temperature. Shivering produces heat in the body. Once the temperature goes up, the person often feels warm. When the fever goes away, the person may start to sweat.


There can be many causes of fever. Sometimes, the reason can be very difficult to find. Your caregiver may have to do numerous tests to track down the reason.


  • Medication may be used to control fever.

  • Do not use aspirin because of the association with Reye's syndrome.

  • If an infection is suspected to be causing the fever and medications have been prescribed, take them as directed. Finish the full course of medications until they are gone.

  • Sponging or bathing in lukewarm water can cool the skin and reduce body temperature. Ice water or alcohol sponge baths are not as effective as lukewarm water and should not be used.


  • Continue to eat normally.

  • Drink enough fluids to keep urine clear or pale yellow.

  • Broths, decaffeinated tea, decaffeinated soft drinks, and oral rehydration solutions (ORS) can help replace fluids and electrolytes.

  • Keep all follow-up appointments as directed by your caregiver.

  • Weigh yourself once a day. Write down the weights and bring them to your follow-up appointments to review with your caregiver.


  • You or your child is unable to keep fluids down.

  • Vomiting or diarrhea develop or are present and become persistent (continued).

  • There is excessive weakness, dizziness, fainting or extreme thirst.

  • You have a fever or persistent symptoms for more than 72 hours.

  • You have a fever and your symptoms suddenly get worse.