On Friday we spoke about a case of fever of unknown origin or FUO.
The original definition of FUO as proposed by Petersdorf and Beeson in 1961 required the following criteria:
(1) Fever of >38.3 on more than one occasion
(2) Fever for more than or equal to 3 weeks
(3) No etiology discovered after at least 1 week of inpatient investigations

However, the definition has evolved with the increasing population of patients with HIV and neutropenia and the ability to perform investigations in an outpatient setting. The following is a new definition proposed in 1991 by Durack and Street:

Hayakawa et al. Am J Med Sci. April 3. 2012 Epub ahead of date

A thorough history, including onset, duration and nature of the fever, is crucial. While the pattern of fever has not been found to correlate significantly to the etiology, two patterns have been described: (1)Pel-Ebstein (fever for 3-10 days on, then 3-10 days off, typical of Hodgkin's lymphoma) (2) Typhus inversus: reversal of normal diurnal pattern seen in TB.

In many cases the etiology can be narrowed down based on the patient's social history. Including the patient's country of origin, vaccination status, recent travel history (where exactly), animal or insect exposure, recreational activities (gardening or swimming in fresh water), sexual activity, use of recreational drugs. The most at risk population for acquiring an travel related infectious disease, are those who immigrated long ago and return to their country of origin to "visit friends and relatives" (VFR), as these people tend to be the population who take the least precautions.

The following is a table of common causes of FUO:

Hayakawa et al. Am J Med Sci. April 3. 2012 Epub ahead of date

For a comprehensive approach to FUO refer to the following interesting articles:
http://archinte.jamanetwork.com/article.aspx?articleid=215227