What are the different reasons for an unexplained fever in a 25 year old?

4 categories. If the fever (t>100.5) has been present for 3 weeks or more and has defied reasonable attempts to explain, the most likely cause of the fever will eventually be "unexplained." another 20% of patients will eventually be found to have an inflammatory cause, another 20% will be found to have an occult infection and 10% will be found to have a malignancy.