A fever of unknown origin (FUO) is used to describe a temperature that has been raised for at least three weeks without a possible cause having been found despite lots of tests being done at a hospital. Unlike a common fever, the causes of FUO are harder to find. A list of some of the causes of an FUO is given below.

Causes of a fever of unknown origin

  1. Infections
  2. Malignancies
  3. Autoimmune conditions
  4. Miscellaneous

Patient subtypes

There are some types of patients are particularly prone to some specific infections. These include:

  1. Hospitalised patients: Infection of the veins, pulmonary embolism (PE), bowel infections, and drug-induced fever.
  2. Immune-compromised patients: In most of these cases, the fever is caused by opportunistic bacterial infections. These patients are usually treated with broad-spectrum antibiotics to cover the most likely bugs.
  3. HIV associated: HIV itself can cause a fever that is often hard to identify and could be an FUO. However, HIV also leaves a patient open to certain types of infections including: Mycobacterium avium-intracellulare complex, Pneumocystis carinii pneumonia, and cytomegalovirus.


  1. Bor DH. Approach to the adult with a fever of unknown origin [online], UpToDate, 2006. Available at URL: http://www.uptodate.com (last accessed 24/08/06)
  2. Bor DH. Etiologies of fever of unknown origin in adults [online], UpToDate, 2006. Available at URL: http://www.uptodate.com (last accessed 24/08/06)
  3. Kumar P, Clark M. Clinical Medicine (Fifth Edition). London, Saunders, 2005.
  4. Murtagh, J. General Practice (Third Edition). North Ryde, McGraw-Hill Australia, 2003.

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