A lot of virus infections are unfortunately like that too. Few years ago I had a fever that wouldn't go away, night sweats, no appetite, nothing......finally got admitted to a hospital with an infectious diseases department, they ran about 40 different tests.....all negative. Did few dozen more, this time on all kinds of tropical/exotic diseases.....all negative. Few weeks of back and forth while I was still
very unwell, losing a lot of weight. The doctor looking after me was suspicious it's actually HIV that's not coming up on the tests for some reason, repeated the tests like 3 times just to be super extra sure.....eventually they just said "it's some kind of virus but we don't know what, so we have no idea - go home and rest, it should improve within few weeks but in the meantime we have no solution for you".
Couple weeks later several of my joints got really swollen which led to them testing me for a human Parvovirus-B6 infection, and indeed, that's what it was - but it was just a lucky guess by the doctor there, he said they don't normally test people in my age group for it because it's incredibly rare for adults to get infected or show any symptoms, but lucky me, I was in that 0.000001% group that not only got infected by also had severe symptoms. But even then it was just "ok, we know what it is but there is nothing we can do to help, it should go away, oh and btw there is a 5% chance you will continue getting symptoms for years if not forever". Well it's been 4 years and I'm still fighting it, so...........