Here's a theory for outbreaks (Those with more medical/biology knowledge than I have are welcome to tell me I'm full of it.

)
Suppose XMRV is transmissable via respiratory secretions,
but not strongly so. Other than direct blood-to-blood contact, the infection rate is 1 infection to 10 exposures
at most with HIV. Suppose again, that XMRV infection rate is 1 infection to 100 or 1000 exposures and that respiratory secretions are only infectious during some stages of the infection. Then most people's chance of getting infected is relatively small, even when they've been exposed.
We know that a number of factors can affect the immune system in such a way that it doesn't perform optimally -- stress, infection, etc.
Now imagine a small close community where people are together for large periods of time -- schools, hospitals, orchestras (wind instrument players get extra credit for sharing their respiratory secretions with everybody around

). It takes 1 person with XMRV in a highly infectious stage
and a group-wide immune function reducing condition (a bad flu, HHV-6A) to spread the infection much more than might normally be seen. This would be especially true if the more infectious stage of XMRV was early in the infection. Then you might have several infectious people in close quarters at the same time.
In summary, I'm speculating that it may take XMRV in a more infectious stage
plus an overlapping virus
plus close quarters to make an outbreak.
I know, by that logic we should see more significant other cases...... *sigh*
But are there any huge flaws in my outbreak theory?