What Is a Fever of Unknown Origin or "FUO"?

QUESTION: Can you please explain the use of the letters "FUO" in describing
an illness. I am sure it has nothing to do with "UFO" but my curiosity has
been provoked, and I hope you will be able to provide me with the answer I
seek. Thank you for your help.
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ANSWER: "FUO" stands for Fever of Unknown Origin, not for UFO or Unknown
Flying Objects. In this age of superb diagnostic abilities on the part of
physicians and relatively new aids to diagnosis--such as CT scans, magnetic
imaging and a whole host of other laboratory tests, there are fewer fevers for
which the cause cannot be determined. Yet, some FUO's occur. They can be of
real concern especially in older patients, the poorly nourished and in those
whose bodies have decreased natural abilities to fight disease.
FUO's are fevers (temperatures elevated above the normal) that last for
several weeks and for which patients and their doctors can find no cause.
Medically, before a patient is diagnosed as suffering from FUO, three points
must exist: (1) fever must have lasted for one to three weeks; (2) body
temperature must have been greater than 101 F on several occasions; and (3)
diagnosis of the cause of the fever has not been possible after one week.
Normal human body temperature ranges from 96.8 to 100.4 F; the lowest
daily temperatures usually occur in the early morning upon awakening, while
the highest are found in the late afternoon or early evening, and following
exercise. Persons whose usual body temperature is at the upper end of the
range and in whom temperatures are taken late in the afternoon may appear to
have a low fever when, for them, it is really normal.
Yet, when a fever for which no cause can be found persists for several
weeks, the sufferer and his/her doctors often become quite anxious.
Through careful diagnostic procedures, many and possibly most, FUO's
prove to be caused by some sort of bacterial infection (possibly tuberculosis,
gonorrhea, appendicitis, abscessed teeth, fungus, or sinusitis to name but a
few). In addition there are certain virus-caused illnesses, tumors,
connective tissue diseases (partial list includes: lupus, rheumatic fever,
and rheumatoid arthritis), endocrine diseases (certain thyroid malfunctions
among them), alcohol, drugs (both those prescribed to treat other conditions
and those that are illegal), and/or psychological conditions.
Among people most prone to FUO's are: the elderly, travelers, recent
immigrants, homosexuals, patients who have undergone surgery recently, those
at risk for AIDS, and those who bodies cannot fight disease in a normal way
because of illness or treatment (such as cancer and chemotherapy).
Nevertheless, whatever the reasons, prolonged fevers are serious and are
cause for concern. Young patients who are otherwise healthy, but with fevers
slightly above the normal range that continue for sometime with no detectable
reason even after a full series of diagnostic tests, usually do relatively
well in spite of their FUO's. Some of these fevers disappear in time; in
others, fevers continue on and off over long periods of time without
developing into a serious disease. Such patients usually remain under close
medical observation, to be sure that the situation is not the beginning of
another, treatable infection.