As we know studies showing a strong correlation between XMRV and CFS have been published. And in that study, 3.7% of the healthy controls were also positive. Here's my hypothesis of how all the dots are connected: XMRV is a virus, which is in the same narrow class of viruses as HIV. And as we know, what HIV does to the immune system is to "break the door open" for many other infections. Otherwise normal infections can and do take lives of some of the HIV patients. But that doesn't happen with CFS patients. They don't die from normal infections, although normal infections can be harder to tackle for them... What I think happens when someone gets a XMRV infection is that they don't get a broad immune deficiency - like with HIV. But what they get is a more narrow, and specific immune deficiency. So when asked which test to go for, Dr. Peterson replies that "the best biomarker for CFS is the Low NKCell function test". Which is a small part of the immune system. I think that the 3.7% of the healthy controls in the XMRV and CFS study, have the same specific immune deficiency that the others had. But since the only had that, they didn't become ill. Immune deficiency + specific infection = CFS I think that what happens when someone becomes sick is that they get a specific infection which normal people would handle well. But with the specific immune deficiency caused by XMRV, this specific infection can really become serious. And when they get that infection, they get symptomatic - meaning the broad symptom package that ME/CFS is.