OK, I've read much of it. This will be a crude once-over synopsis.
Basically, these people discovered that certain retroviruses including XMRV encode a piece of a molecule with a very potent immunosuppressive function. They made mutant viruses which lack this function, and those viruses basically could not infect mice.
To give a little more detail for those with some background, it seems the Env (envelope) protein of a group of retroviruses including XMRV is not just an envelope protein. It is within the Env that this group discovered this potent immunosuppressive domain which.
The weak mutant virus without the immunosupressive domain fails to infect normal mice. It does better when the mouse's NK and CD8 cells are hampered artificially. Therefore, the new immunosuppressive domain somehow puts a damper on these immune cell types doing their thing. Which is of course to kill varmints like retroviruses et al.
The weak mutant virus induced a much stronger immune response. Why Mister, lymphocytes were spewing much more IFNg and other fine molecules that would, when produced in sufficient quantities, kill stuff and resolve an infection.
So, if a drug can inhibit thise immunosuppressive molecule domain robustly, that alone might plausibly bring the immune system around, to the extent that it would suppress XMRV to non-pathogenic levels and you would feel fine. What's not clear, apparently, is what that little domain is and does. Therefore, it doesn't necessarily have to be druggable, but my half-informed guess is that the chances are probably pretty fair. Some types of interactions that *might* be occurring here, namely protein-protein interactions, can be relatively difficult to inhibit well with drugs -- but that doesn't necessarily mean it can't be done. Especially when there is a huge amount of potential revenue to be made, which means we might see very large and very flush research projects in several firms (almost all drug discovery is done by the private sector, which has a lot more dough than government entities like NIH).