The problem with finding microorganisms in certain situations is determining what organisms are causing disease and what organisms are just 'hanging out.' Sometimes this is hard to determine. I did find some old research comments ( several years ago - at a university site) that XMRV was considered pathogenic for humans and appropriate containment should be used. Sorry, can't find original source.
Here is a bit of obscure viral info. There is a lactic dehydrogenase virus described in 1965 by Abner Notkins, National institute of Dental Research. ( Bacteriol Rev 1965 June; 29(2): 143-160) " From these studies, it can be seen that the widespread contamination of tumors, viruses, and animals with LDV adds considerably to the already complex problem of extraneous mouse viruses and of the interpretation of experiments in which LDV is a contaminant." So, the presence of one virus may affect the ability of other viruses to cause disease.