Another thought. It's clear that the WPI are working with the idea of cofactors as triggers for XMRV to turn into CFS. What if the order in which people get these viruses or other life events is important?
eg. I got CFS at age 37 after a bout of EBV. But my mother has fibromyalgia, father just died of prostate cancer, mums father died of prostate cancer and her youngest brother has just been diagnosed with it. Based on the current WPI hypothesis, if I had to guess, I'd say I was probably carrying XMRV all my life causing no harm until I got EBV.
But what about a scenario where someone has EBV as a child, is thus immune, then later contracts XMRV? Maybe they would be OK and could be a carrier with one less thing that can convet XMRV to CFS?