It is already known that drinking alcohol (beer, wine, liquor) and smoking, increases a woman’s risk of breast cancer.
A virus found in beef and dairy products seems to be another possible risk factor for breast cancer, according to a case-control study involving 239 donors of breast tissues.

The study published in PLoS One found that BLV (Bovine Leukemia Virus) in breast tissue was strongly associated with breast cancer. It was present in 59% of the women with breast cancer, comparing to 29% in the control group.

In the USA, about 38% of cows used in beef production, and 84% of cows used for dairy are infected with BLV.

Other studies on diary products

There are other studies made earlier that is pointing to the same: A link between breast cancer and dairy products. According to Dr T. Colin Campbell, PhD, out of 144 human studies that showed statistically significant associations of fruits and vegetables with cancer, all of them showed a protective effect. (World Cancer Research Fund Report 1997)

Dr T. Colin Campbell’s was involved in a big nutritional study called the China Study (or the China-Cornell-Oxford Project), over a 20-year period. This study concludes that animal proteins (and in particular casein found in milk) is linked with cancer, including breast cancer. (Campbell and Campbell 2005, The China Study, pp. 87–88)
They also found that plant based foods have the opposite effect – a protective effect.

It looks like the safest is to stick to the plant based diet, that God gave to man in the Garden of Eden (Genesis 1:29)

Buehring GC, Shen HM, Jensen HM, Jin DL, Hudes M, Block G. Exposure to bovine leukemia virus is associated with breast cancer: a case-control study. PLoS One. 2015;10:e0134304.