Thanks Jemal for posting the link to the recent paper concerning downregulation of APOBEC3 by XMRV. I was hoping that somebody would do so. I recall that there was earlier evidence of this effect published a year or more ago, and that it's still an open question as to how effectively APOBEC3 restricts XMRV in vivo. It's surprising to see that argument still being repeated as evidence against any potential danger from PMLVs when the downregulation of APOBEC3 by XMRV has been reported here frequently, and that evidence has never been challenged here as far as I have seen.
Tony, I find this argument of yours rather interesting...
The environment is stock full of viruses, we get them all time from animals we are killing, butchering and eating and we have eaten animal for over a million years, probably much longer. We injured and have been injured by other animals for a very long time and had lots of chances of exchanging viruses. If mouse viruses got into humans, it has already happened 10.000 years ago. The main threat are pathogens from close relatives (SIV->HIV).
I agree with your premise that 'bushmeat' theories of viral transmission can reasonably be expected to have exchanged viruses between animals and humans of the order of 10,000 years ago, and on that basis I consider that any new viruses and diseases that have occurred in humans within a much more recent timeframe are highly unlikely to be explained simply by 'bushmeat' theories. Your argument seems to say the same thing - but in that case I'm confused as to why you make an exception for close relatives (SIV -> HIV), and surprised that this understanding doesn't focus you on wondering how new human diseases like AIDS and ME have emerged within such a historically recent timeframe.
Doesn't it seem dubious, by your own argument, to accept that SIV jumped from monkeys to humans within the last century but not before? Isn't it more likely, given the very clear timeframe of the emergence of AIDS as a disease within the last century at most, that the emergence of HIV is in some way a consequence of modern conditions? I know that the vaccination theory of the emergence of AIDS is considered to be 'debunked', but I don't personally find the evidence of that debunking at all compelling, and there remains the suspicious fact that Koprowski's polio vaccination was grown in tissue cultures taken from macaque monkeys before being administered to about a million people in Burundi, Rwanda, and what is now the Democratic Republic of the Congo, shortly before the emergence of HIV in those areas.
That's a little way away from your point about mice, though, so on the specific point about mice, it's important to note that many strains of mice came into existence within the last century or two due to breeding as pets, and the specific strains of mice we are interested in (nude mice, specifically) have been widely used as laboratory mice due to their suppressed immune response. The emergence of novel (retro)viruses like XMRV during laboratory experiments on selectively-bred immune-compromised mice is quite clearly a reality which nobody is disputing, and there is no guarantee at all that these novel viruses and retroviruses cannot be pathogenic in (at least some) humans.
It would be very rash indeed to make assumptions that any such novel viruses and retroviruses (a) cannot infect humans, and (b) must behave in the same ways (eg re: cancer incidence) as the two human-infectious retroviruses we are currently aware of. There just isn't enough evidence to rely on those two assumptions, and even though there's now no evidence on the books of XMRV infection in humans, the more general risk of infection of humans from laboratory-created retroviruses is very clearly a genuine risk and a genuine possible explanation for novel (and potentially vaccine-mediated) human diseases - especially so since the infection of animal vaccines with previously-unknown retroviruses, and their transmission across species through vaccination, is also accepted as scientific fact. The most that can be said against this entirely plausible hypothesis is that there is currently no clear evidence (on public record, at least) that this scenario has ever occurred. So it does remain merely a hypothesis, but it's unquestionably a hypothesis that demands extremely close scrutiny.