If you get a diagnosis at all, it's going to be a catchall for "idk" like fibromyalgia, CFS, allergies or it's filed away as psychosomatic.
There are probably a lot of syndromes caused by latent infections from virii like EBV, various herpes, HPV, etc that don't go properly diagnosed or treated if they can be treated at all.
Also, it's been found that some viral infections retreat to tissues where their detection might be difficult/impossible without biopsy. You saw this with, for example, Ebola, where survivors could test negative for the virus, but it would still survive and replicate in different isolated tissues like those in the eye.
There are probably a lot of syndromes caused by latent infections from virii like EBV, various herpes, HPV, etc that don't go properly diagnosed or treated if they can be treated at all.
Also, it's been found that some viral infections retreat to tissues where their detection might be difficult/impossible without biopsy. You saw this with, for example, Ebola, where survivors could test negative for the virus, but it would still survive and replicate in different isolated tissues like those in the eye.