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Fever of Unknown Origin


Fever of unknown origin is a higher body temperature with no clear cause, even though there has been at least one or two weeks of testing.


The cause of this fever is unknown. In some people the cause may never be known.

Factors that may make it difficult to find a cause include:

  • A common illness that does not have the usual symptoms
  • Illness, whose other symptoms appear later
  • Illnesses who may have a delayed positive test
  • Person is unable to communicate about other symptoms such as an infant or someone in a coma
  • Genetic condition that causes periodic fevers—very rare

Risk Factors

Since the cause of FUO's is unclear, there are no specific factors that increase your chance of this fever.


A fever is generally considered a temperature over 101 degrees Fahrenheit (38.3 degrees Celsius) but the exact number can vary. A fever of unknown origin may be consistent or occur sporadically.

The fever may also be accompanied by chills, sweating, or other symptoms that are caused by the underlying illness.


Your doctor will ask about your symptoms and medical history. A physical exam will be done. If there is no clear cause, your doctor will begin to narrow possibilities. The first step is to ask about your recent history such as:

  • Were you traveling abroad?
  • Were you hospitalized?
  • Is your immune system damaged?
  • What medicines are you currently taking?

Based on your history your doctor may then order some tests to look for possible causes. Tests may include:

  • Blood, urine, and other body products testing
  • Images of internal organs and structures with x-rays , CT scans, MRI scans, or ultrasound
  • Biopsy of suspicious tissue
  • Endoscopies to examine the inside of the lungs, intestines, or sinuses
Endoscope in stomach
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Treatment for the fever may not be necessary since a fever is a normal part of your immune system. Lowering the fever with medication may make it harder for your body to fight the infection, if one is present. Your doctor may recommend medication to lower the fever if it is extremely high or causing other health related problems.

If an underlying condition is found, treatment will be based on that illness.


Since the cause is unclear there are no steps to prevent FUO.

Revision Information

  • American Academy of Family Physicians


  • American Academy of Pediatrics


  • Caring for Kids


  • Health Canada


  • Roth A, Basello G. Approach to Adult Patient with Fever of Unknown Origin. Am Fam Physician. 2003 Dec 1;68(11):2223. Available at: http://www.aafp.org/afp/2003/1201/p2223.html. Accessed May 20, 2013.

  • Fever of Unknown Origin (FUO). EBSCO DynaMed website. Available at: http://www.ebscohost.com/dynamed/what.php . Updated November 13, 2012. Accessed May 20, 2013.

  • Gelfand JA, Callahan MV. Fever of unknown origin. In: Kasper DL, Braunwald E, Fauci A, Hauser S, Longo D, Jameson JL. Harrison's Principles of Internal Medicine . 16th ed. New York: McGraw-Hill; 2005: 16-121.

  • Goldman L, Ausiello D, eds. Cecil Textbook of Internal Medicine. 23rd ed. Philadelphia, PA: Saunders; 2008.