A fever without a known cause that that lasts a long time.
Fever of unknown origin is a lasting fever without a known cause after testing for common causes. Possible causes are an infection, cancer or an inflammatory disease that isn't passed from one person to the next.
The definition of fever of unknown origin is having a fever higher than 101 degrees Fahrenheit (38.3 degrees Celsius) on several occasions, fever lasting at least 3 weeks and a cause of fever that isn't certain after testing.
Treatment depends on the underlying cause once it's found out.