http://www.the-scientist.com/?articles.view/articleNo/32990/title/Ancient-Viruses-Wreak-New-Havoc/
"...Though researchers had suspected that ERVs could reactivate and recombine, the study provides direct evidence of such an event, says immunologist David Markovitz of the University of Michigan, who was not involved with the study. “It’s extremely interesting that the mice activate or reactive their expression of ERVs in the absence of antibodies, ...
Previous data have indicated that signals from bacteria in the mouse gut could lead to increased EVR expression, but that antibodies usually block those microbial signals. Thus, Kassiotis’s team hypothesized that in the absence of such antibodies in their immunodeficient mice, invading microbes could activate ERV expression. Indeed, when placed in germ-free environments, or fed acidic water that would limit their gut microbes, the immune-deficient mice experienced less viral reactivation…"