This one goes with the pig study.

Porcine Circovirus type 2 (PCV2) causes apoptosis in experimentally inoculated BALB/c mice

This one that shows the whole virus isn't needed to cause damage

http://www.ncbi.nlm.nih.gov/pubmed/15956572

Edit:
And I just found this one - which was absolutely shocking to me and has now given me a huge conflict of bias because it's become very emotive for me now. My father died from BOOP, as a pre-teen I slowly watched him die of hypoxia - going slightly mad - over three years. We were told it was a rare disease - but apparently it's not so rare in pigs infected with PCV2 which is a causation for the disease - why the hell would I ever risk sticking particles of this disease in my children just to stop them getting gastro, an illness neither has ever had despite not being vaccinated for it.
http://www.ncbi.nlm.nih.gov/pubmed/20976305