I just watched a video on youtube in which a pain doctor at stanford university speaks about fibromyalgia (which is what I have): http://www.youtube.com/watch?v=jtc2JARVpPw In a few minutes of this long conversation he speaks about their studies regarding Low Dose Naltrexone. He says that microglia cells are cells that are constantly scanning for infectious agents and cell damage. They are in our brain and spinal cord. They get heavily activated during an infection and during cell damage. When they get activated they move from their areas to go and attack where the problem is - and they are producing a number of chemical factors in the brain and in the spinal cord (one such chemical factor are proinflammatory cytokines). He says that these chemical factors can ultimatley increase pain sensitivity and lead to what people experience and call "illness symptoms" (remember when ME/CFS was called "Yuppie flu"?). He says that what we believe is happening in chronic pain is that these microglia cells are not going back into their normal state and they continue to release these chemical factors. He says that Naltrexone, specifically when taken in low doses, block a specific receptor (called "toll-like receptor 4") on the microglia, and it prevents the release of these proinflammatory agents that the microglia is usualy releasing when it's activated. By the way, their first small study on 10 fibromyalgia patients has been successful (7 of them had significant improvement) and eight days from now, in the 24th of April they are going to present, as far as I understand it, the results from their phase II trial, the as far as I know contained 60 patients and was placebo controlled, in the LDN conference in glasgo ( http://www.bigonldn2010.com ). By the way, ME/CFS is going to be a major issue there. So, coming back to the theory. I think a possible theory for causation by XMRV might be: Let's say that XMRV sits in our brain (or spinal cord?). The microglia gets activated - because there is an infection, even if it's not doing anything by itself (I suppose the microglia can be activated in that case too), and it releases the chemicals we talked about. The chemicals doesn't get XMRV out of our brain, so XMRV continues to sit there and the microglia continues to be activated and release the chemicals - and the chemicals are what causing our pain, or as the doctor said in his presentaion, "feeling of being sick". I guess, and correct me if I'm wrong, that these microglia, when activated, can somtimes cause symptoms that we see in fibromyalgia, sometimes cause symptoms the we see in ME/CFS, and perhaps even symptoms that we see in irritable bowel disease and in other diseases that might actually be CNS (central nervous system) diseases. There is something that I don't know to explain, and its why the healthy people that carries XMRV don't get sick. Perhaps it has something to do with genetics? I definitley don'y say that it's the theory that would turn out to be the right one, but it seems to me like a possible theory... Anyway, people, what do you think? Edit: By the way, the part about micgorlia and Low Dose Naltrexone in that video starts in 42:40 minutes, and ends in 50:21 minutes.