Looking at XMRV, and seeing the mention of low copy numbers, how could such a virus cause such nasty disturbances in the immune system. As a scientist, I have to take an analytical point of view - a protein called APOBEC3 has been shown to strongly inhibit retroviral replication. APOBEC3 is however only expressed in certain cells - testicular germ cells most mature blood cells in particular. However it is only expressed in low numbers, or not at all in peripheral tissues. That probably explains why it is found in such low copy numbers in blood cells and seminal plasma. However, that does not mean one should engage in unprotected sex with someone with Fibromyalgia or CFS, even though XMRV is present in low copy numbers, ***IT IS STILL INFECTIOUS***. Tonsilar tissues have been shown to be particularly prone to infection by XMRV.
Rather than simply giving up, it means we have to look elsewhere. By elsewhere means brain, nervous system, lymphatic tissues, bone marrow, tonsils, prostate, spleen, muscle. However, it would not be practical or ethical to biopsy brain or nervous tissue - which leaves the two best choices - bone marrow biopsy which can be done in a doctor's office, and muscle biopsy (in particular from tender points of fibromyalgia patients). Both can be done with a needle, with only slight discomfort.