The thing about viruses is everyone has them. HHV6 and CMV are often contracted in early childhood. all of the herpes viruses stay in the body until you die and in the healthy population, they will reactivate every now and then without the person knowing.
@Anana After spending a good deal of time researching during our son's treatment (treated by Dr Lerner), I learned some interesting things about viruses and immunity.
@Kati is correct in saying that almost all of us carry certain viruses in our bodies including HHV6, EBV, VZV and more. Most if not all the test I am familiar with look for antibodies (the bodies response to having a viral infection). Problem is the level of antibodies in the blood (titer) is not an indication of an infection or re-activation. That has to be supported by other clinical findings. The same logic can also be applied to Lyme disease.
So in theory you can test positive and not have an infection/re-activation (false positive) and you can test negative and have an infection especially early on (false negative). This is why the importance of finding a physician who is very good at clinical exam and very good at clinical reasoning and applying the appropriate treatment. These are skills that are learned primarily by experience.
We did see Dr Lerner for our son's treatment which was successful. He "tested positive" for HHV-6 and later for EBV. Dr Lerner's treatment regimen was primarily anti-virals. Being a Physical Therapist and watching my son go through this treatment, it seems to me that the anti-viral was doing more than just "killing virus cells". This is also the opinion of Dr. Davis's recent report which included some preliminary results of testing the effect of Valcyte on the metabolism of blood drawn from a ME sufferer.
I think there is a Neuro-physiological affect of some anti-virals either directly or indirectly. For support of this theory, there is support for the use of Amantadine (an anti-viral) in Post-Concussive Syndrome (which looks a lot like ME). I am not saying that viruses do not play a role in ME as I am confident that my son's came on following a viral infection. I am saying that I would view antibody tests with a bit of skepticism as far as whether the results indicate a reactivation is creating a crash. Does not mean you should ignore the symptoms by any means though.
Research is mounting that ME is a bio-molecular/metabolism dysfunction triggered by any number of insults to the body (Virus, Lyme Disease, Bacterial Infections, Allergic Reactions, Severe Stress/PTSD and others we may as of yet not know). It is accompanied by extreme levels of Neuro-inflamation in the brain and likely has a Neuro-immune/protective response gone "out of control" (maybe a false feedback loop?) at its core.
Sorry for the lengthy post! Just learned a lot going through this with our son. The good news is it may not currently be related to Viral infections. The challenge is to figure out why the response is triggered and then sustained as well as what we can do to treat the bio-molecular fallout from it and eventually the triggering mechanism itself. You are in very good company here on PR!