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)
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?
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