It fits perfectly Let say you have billions of XMRV virons or virus partials swimming around in your blood busily infecting the cells it likes best. What are the chances that a virus will actually infiltrate a sperm or an egg cell? Fairly small actually since both the egg and sperm are coated with a specific protein that protect them and they are both in very protected areas of the body. Now what are the odds that the infected sperm or egg would go on to become fertile and produce an offspring? Considering how many eggs pass through without fertilization and that only one sperm out of millions will fertilize any one egg the numbers become inestimably tiny. That's what is necessary for virus or retrovirus to become Endogenous. It happens, but it takes a really long time, minimum a hundred generations. and . . . The retrovirus itself must change and be able to pass barriers around ova and sperm production sacs. Plus the virus must be able to pass through the barrier of the sperm or egg itself and that's a whole complex protein structure in itself. Right now however the versions of XMRV that are being passed around are stimulated by cortisol and due to the envelope around the virus are able to slip in undetected in T, B and NK cells for sure and probably some others that we don't know about yet. But the changes necessary for it to infect the "germ line" (egg and sperm) are not there at this time. thus; There's no evidence it can become Endogenous at this time. But we are passing live virons down via blood, and milk to our children. And we are passing it via sexual transmission to each other. These scientist are talking to other scientist and figure that all of the middle parts are understood so it's left out of the writings. The other confusing thing is that this is a new retrovirus. Scientist deal in facts and won't make a statement that they can't back up. They get major slapped down if they do. So they leave out any thing that they don't have iron clad, double blind study to back up. Heck there's doctors still disputing the mechanisms for HIV. Have you ever been asked the math question ; If you start out with one penny and you double the number of pennies each day how many day's will it take you to reach a million dollars? Now think about it like this; If you have one person with XMRV and that person passes it on to one other person. Each of those person pass it on to one other person. It's in effect like doubling the pennies only it takes much longer. So you can create a pandemic long before the retrovirus has a chance to fade into the human genome. I hope that was clear. If not say what's not and I'll try to clear it up some more.