by Polsdorfer R
(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
  • Illness with symptoms that only appear later
  • Illness that may not show up on tests until later, called delayed positive test
  • 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.


You will be asked about your symptoms and past health. 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.


Family Doctor—American Academy of Family Physicians 

Healthy Children—American Academy of Pediatrics 


Caring for Kids—Canadian Paediatric Society 

Health Canada 


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:

Fever of unknown origin (FUO) in adults. EBSCO DynaMed website. Available at: . 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:

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