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Fever of Unknown Origin

  • Ricker Polsdorfer, MD
Publication Type:


Fever of Unknown Origin

(FUO; Pyrexia of Unknown Origin)


Fever of unknown origin (FUO) is a fever with no clear cause even after testing for at least 1 to 2 weeks. A fever is often defined as a temperature higher than 101 °F (degrees Fahrenheit)/(38.3 degrees C).


The cause of this fever is unknown. In some people, the cause may never be known. Finding a cause may be delayed if there is:

  • A common illness that does not have the usual symptoms
  • An infection that is often hard to diagnose
  • An illness with symptoms that only appear later
  • An illness that may not show up on tests until later, called a delayed positive test
  • A genetic condition that causes fever (rare)

Risk Factors

The risk of an illness is often based on the cause. There are no risk factors for FUO since the cause is unknown.


The fever may be steady or come and go. Chills, sweating, and other symptoms may also be present. They can vary based on the cause.


The doctor may ask questions to look for possible causes. A physical exam will be done. Your doctor may ask questions to look for possible causes. Questions may include:

  • Have you traveled?
  • Have you been hospitalized?
  • Is your immune system damaged?
  • What medicine are you taking?
  • Have you been near anyone who has been ill?
  • Have you ever been around someone with tuberculosis?

Tests to look for cause may include:


Medicine may be used to lower fevers that are very high or causing other problems.

Treatment may be changed once a cause is confirmed.


It is hard to prevent FUO since the cause is not known.





  • Cunha B, Lortholary O, et al. Review of fever of unknown origin: clinical approach. can be found in Am J Med 2015 Oct;128(10):1138. Accessed at: https://www.ncbi.nlm.nih.gov/pubmed/26093175.
  • Fever of unknown origin (FUO) in adults. EBSCO DynaMed website. Available at: https://www.dynamed.com/approach-to/fever-of-unknown-origin-fuo-in-adults/. Updated January 13, 2017. Accessed February 6, 2020.
  • Varghese G, Trowbridge P, et al. Investigating and managing pyrexia of unknown origin in adults. BMJ 2010;341. Accessed at: https://www.ncbi.nlm.nih.gov/pubmed/22312655?dopt=Abstract.


  • David L. Horn, MD, FACP
Last Updated:

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.