Mice share a lot of biological similarity with humans too, which is one reason they are so much used in research. I see your argument more clearly now, but it's just an opinion and an assumption really, not a (dis)proof of anything. If it was completely unfeasible that a virus could come from mice to humans, why was the XMRV theory taken so seriously by so many? Even Dr Coffin took it very seriously for quite a long time. It isn't implausible. There's very little evidence of such a disease affecting so many people prior to 1934. I also find the early reports, particularly the reports from 1934 and 1955, quite strong evidence - the physicians then were baffled by these outbreaks and had never seen or heard of anything like it before. I think if we weren't looking at a modern phenomenon, there would be better historical evidence. It's hard to imagine that 0.5% of the population have always had this disease and nobody noticed it before.