It's well known that a HIV infection makes the CD4 count go down. Taking a CD4 test (it's a cheap blood test) is a important marker to see how well the patient responds to treatment. CD simply means "Cluster of differentiation". All known cell surface molecules which appear on white blood cells are given a CD number. When someone gets a HIV infection, the virus gains entry to the white blood cells (T-cells to be specific) through the CD4 receptor. When the infections is serious, this viral attack can be seen simply by doing a CD4 count. As you can see here (this is a graph of a untreated HIV infection). When the viral load goes up, the number of CD cells goes down. So to sum it up: HIV gains entry to the T-cells through the CD4 receptor. What is not so well known is that very many of the CFS patients have low CD57 counts. The normal range is from 60-360, and many have levels of <50. As I've understood it, the CD57 has much of the same relation to Natural Killer Cells, as CD4 has to T-cells. And with CFS, these Natrual Killer cell problems are not uncommon: - Malfunctioning of natural killer (NK) cells has long been recognised as an important factor in the development and reoccurrence of cancer, and has been documented repeatedly in CFS patients (PMID 20032425). So what I would like to see would be tests of CD57 done prior and after treatment has begun. It's cheap, and done by labcorp (and many others). It's so common in all of the syndromes that have been linked to XMRV, to have low CD57 counts... Examples are CFS, FMS, atypical MS.