I googled it because I was very skeptical. This is not the paper but the press release to what I think the above commenter was referring to: http://news.berkel

I googled it because I was very skeptical. This is not the paper but the press release to what I think the above commenter was referring to: http://news.berkeley.edu/2015/09/15/bovine-leukemia-virus-breast-cancer/ http://news.berkeley.edu/2015/09/15/bovine-leukemia-virus-br... A PLOS one paper showing evidence of this virus in 59% of breast cancers sampled compared to 29% in non cancerous breast tissue of other women. N size was 239. I think this deserves future study to see why there is this apparent association but the above poster's claim that this shows that it 'causes 39% of breast cancer' is simply not credible at this time.