"A pyrexia of unknown origin, commonly shortened to PUO, and also known as a fever of unknown origin (FUO), was originally defined in 1961 as the condition in which the core body temperature is >38.3oC for a period of three weeks or more, with no diagnosis reached after one week of inpatient investigation 1. Setting the period at ≥3 weeks was not arbitrary, but was felt likely to exclude self-limited viral infections, which usually resolve in under 3 weeks 5."
"In modern clinical practice, the term is often used more loosely by clinicians for any case in which a patient presents with a persistent fever without clear cause after a set of reasonable initial investigations. A typical request for a CT chest abdomen and pelvis for such a case may follow along the lines of "PUO, ? source of infection"."
"Another term, inflammation of unknown origin, has been coined for cases of unexplained chronic inflammation without fever. Unsurprisingly, there is a certain degree of overlap between the causes of pyrexia/fever of unknown origin and inflammation of unknown origin 5."