A fancy way of doctors saying that they have absolutely no idea what is causing the fever is PUO- Pyrexia of unknown origin or more commonly termed FUO, Fever of unknown origin.
When doctors come across patients who were admitted for evaluation of PUO, they were forced to put on their thinking cap, generating differentials that could suggest a cause for the patient's on going chronic fever. In reality, they know just as much as the patient at that point of time and I do admit, it takes hell of a gut for a doctor to admit that he doesn't know what is wrong with you or what is causing the fever.
This 24 y.o female patient was referred from the OPD for evaluation for her 2 week history of unsettling fever. The HO is indeed stressed out to find out the exact cause by looking at all the tests he/she ordered