Fever of unknown origin history and symptoms

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: febris continua e causa ignota; febris e causa ignota; febris E.C.I.; fever/pyrexia of obscured/undetermined/uncertain/unidentifiable/unknown focus/origin/source; fever/pyrexia without a focus/origin/source; FUO; PUO


History is the most important step in making diagnosis of FUO, most important components of history are listed below:[1]

History of presenting illness

Proper attention should made in documenting history of presenting illness which is mostly fever, duration of fever, pattern of fever, and associated symptoms may provide clues toward diagnosis and limit unnecessary investigations.

Past medical and surgical history

Chronic medical illness such as tuberculosis, SLE and other autoinflammatory conditions should be inquired as they may be the source of fever. Surgical history is important as post operative complications can cause fever.

Social History

Social history is very important because diseases like infective endocarditis and osteomyelitis are common in drug abuser, Histoplasmosis is common in cave explorers, leptospirosis common in surfers, catch scratch disease and diseases caused by insects are common in people exposed to animals. Some diseases are endemic to certain places hence people should be inquired about their place of origin as all of the aforementioned diseases can be cause of FUO.[2]

Socioeconomic and Vaccination history

Some diseases are limited to poor population due to malnutrition, decreased immunity and living in overcrowded places which increases the chances of contact with the disease source. Unvaccinated people are prone to certain diseases that may cause FUO.

Family history

Genetics play important role in acquiring and response to disease.


Fever is the main symptom and proper attention should be given to the duration, frequency, magnitude, pattern of fever and associated symptoms as some fever patterns are very specific for some diseases and narrows down differential diagnosis.[3]


  1. High magnitude and persistent fevers the most likely cause is infectious.
  2. Relapsing fever the most likely causes are Borrelia burgdorferi, Q fever, Typhoid fever, Brucellosis, Leptospirosis, Colorado tick fever.
  3. Biphasic fever the most likely cause is African hemorrhagic fever.
  4. Fever with relative bradycardia the most likely cause is legionnaires’ disease or drug fever.


  1. Wright WF, Auwaerter PG (2020). "Fever and Fever of Unknown Origin: Review, Recent Advances, and Lingering Dogma". Open Forum Infect Dis. 7 (5): ofaa132. doi:10.1093/ofid/ofaa132. PMC 7237822 Check |pmc= value (help). PMID 32462043 Check |pmid= value (help).
  2. Hayakawa K, Ramasamy B, Chandrasekar PH (2012). "Fever of unknown origin: an evidence-based review". Am J Med Sci. 344 (4): 307–16. doi:10.1097/MAJ.0b013e31824ae504. PMID 22475734.
  3. Cunha BA (1996). "The clinical significance of fever patterns". Infect Dis Clin North Am. 10 (1): 33–44. doi:10.1016/s0891-5520(05)70284-1. PMID 8698993.