Some really interesting developments into our understanding of breast cancer biology are coming from the work of Dr. James Holland, distinguished professor of neoplastic diseases at Mt. Sinai School of Medicine in New York.
In a nutshell, they've been presenting fairly compelling evidence that a virus, the human mammary tumor virus ( HMTV ), a virus 95%+ similar to a virus commonly carried by the common house mouse ( Mus domesticus ), may in fact be the trigger for a great number of human breast cancer cases, maybe up to 1/3. Copies of the gene affected by HMTV appear to be present in high numbers in tissue of women with aggressive forms of breast cancer or those developing during pregnancy or lactation periods.
Fascinating circumstancial evidence supporting this cause-effect relationshiop is the fact that you can draw a map of the planet where these mice flourish and it correlates very closely to areas where breast CA rates are higher. We've traditionally attributed geographic differences in breast cancer to genetics, diet, and the environment but maybe it's something as simple as " where the mice live". The obvious sociological observation that in many instances the poor and underclasses often live in closer proximity to rodents may explain why breast cancer rates are higher in poor neighborhoods (both in America and elsewhere).
The signifigance of this is profound in that we could potentially vaccinate for this much like we can do for the human papilloma virus (HPV) for women's cervical cancer.