My claim is a bit of a stretch, 37% may be attributable. Mechanistic cause is still a bit shaky.

G C Buehring, H M Shen, H M Jensen, D L Jin, M Hudes, G Block. Exposure to Bovine Leukemia Virus Is Associated with Breast Cancer: A Case-Control Study. PLoS One. 2015 Sep 2;10(9):e0134304.

G C Buehring, H M Shen, H M Jensen, K Y Choi, D Sun, G Nuovo. Bovine leukemia virus DNA in human breast tissue. Emerg Infect Dis. 2014 May;20(5):772-82.

K Alibek, A Kakpenova, A Mussabekova, M Sypabekova, N Karatayeva. Role of viruses in the development of breast cancer. Infect Agent Cancer. 2013 Sep 2;8:32.

J Akhter, M A Ali Aziz, A Al Ajlan, A Tulbah, M Akhtar. Breast cancer: is there a viral connection? Adv Anat Pathol. 2014 Sep;21(5):373-81.

G C Buehring, S M Philpott, K Y Choi. Humans have antibodies reactive with Bovine leukemia virus. AIDS Res Hum Retroviruses. 2003 Dec;19(12):1105-13.

J S Lawson, B Heng. Viruses and breast cancer. Cancers (Basel). 2010 Apr 30;2(2):752-72.

(Author unknown) Bovine Leukosis Virus (BLV) on U.S. Dairy Operations, 2007.Veterinary ServicesCenters for Epidemiology and Animal Health. APHIS Info Sheet.

J F Ferrer, S J Kenyon, P Gupta. Milk of dairy cows frequently contains a leukemogenic virus. Science. 1981 Aug 28;213(4511):1014-6.

G C Buehring, P M Kramme, R D Schultz. Evidence for bovine leukemia virus in mammary epithelial cells of infected cows. Lab Invest. 1994 Sep;71(3):359-65.

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-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.