'He' is me. 😮
Sounds reasonable on the face of it. However, medically, a fever of unknown origin (FUO) is an asymptomatic fever - cause unknown. The cause can be quite serious and must be uncovered, but without any symptoms it is a bit of a hunt in the dark. Thus they did a huge number of tests on me - lines of doctors - but did not uncover anything new in my health history.
This is the second time that I have had a FUO in about a year... troubling but certainly not hopeless. :^)